Walls of Constantinople
Following the withdrawal of the Roman legions from the out-posts of the Western Empire and the fall of Rome, the Western nations were left to defend themselves against their barbarous foes as best they might. For many centuries henceforth they were involved in a desperate strife of a kind which allowed little or no opportunity for the development of military architecture. It was in the Byzantine Empire that progress in this direction was made
Here, while the Persian frontier was maintained in the East, the advance of the Goths and Huns was held in check in the north and west. To ensure the protection of Constantinople from the onslaughts of the latter, a great wall, strengthened every sixty yards by powerful towers, was built in AD413 across the west, the land side, of the promontory on which the city stands. Having suffered severely from earthquake, this wall was repaired in AD447, when an outer wall was added and a wide moat dug before the outer wall. The walls are built of stone with a concrete core, and are bonded with brick lacing courses five bricks thick. The inner wall is 15 ft. 6 in. thick at the base, and rises to a great height. The outer wall, 6 ft. 6 in. thick, is constructed with a continuous series of internal arches. There are two wide terraces, or berms, one between the walls and the other between the outer wall and the moat. This noble fortification with its triple lines of defence, the inner and outer walls and the moat, repaired, and in some places altered, later, was a powerful bulwark of defence against the attacks of successive invaders; and even now, shattered by earthquake and neglected, is one of the most imposing and inspiring works of its kind in existence.
The fortifications of Nicaea in Asia Minor, though incorporating much work of later dates, are largely of about the middle of the fifth century. They have been ascribed, recently, to Justinian. But, apart from the fact that they resemble the land walls of Constantinople both in structure and disposition, it is not without significance that Procopius, in dealing with the subject at length, does not mention the fortifications in his descriptions of Justinian’s works at Nicaea. Nicaea is surrounded by a double line of walls, the inner wall of great thickness and height, and the outer lower and less substantial. Both walls are built of stone with brick lacing courses, and are strengthened by towers, placed at frequent intervals; the towers being so spaced that those in one wall stand opposite to an interval in the other.
Plan of Nicaea
A large residential tower on the south side of the city, higher and more powerful than the others, must have been a kind of donjon. It was against this tower, then occupied by the Sultan’s wife, that the Crusaders concentrated their assault in the memorable siege of 1097; and it was not until attack after attack had been made upon it that the tower was eventually brought down, and then only by means of undermining the walls. There are four gateways and three posterns. The walls of Nicaea have withstood many attacks. They repelled the Crusaders again and again and are still in good state of preservation. Of the military works on the confines of the Byzantine Empire those of the fortress of Babylon of Egypt, now called Old Cairo, deserve special attention. This fortress, built in its present form during the latter part of the fourth or early part of the fifth century, stands on the right bank of the Nile at the head of the delta, and not only commanded the river at this strategic point, but also the passage across the river of the great caravan route from North Africa to the East. On the side of the fortress towards the Nile, and directly commanding the passage, are two circular towers, each 90 ft. in diameter and standing 66 ft. apart. These towers are of exceptional interest, not only on account of their unusual design and great size, but also because they stand close together and are designed to correlate with each other. They are constructed of small squared stone with brick lacing courses, stand on foundations consisting of large blocks of stone, and are bonded in with the curtain walls. Each tower consists of two concentric walls, spaced 15 ft. apart, and of eight radial ribs which connect the two walls and divide the intervening space into eight equal compartments. This is a particularly powerful and scientific method of construction, for, while the combined walls have the effective strength of a single wall 28 ft. thick, there is a great saving of material, and the tower is provided with spacious chambers in addition to a large circular room. The heavy ribs, radiating toward the centre, offer great resistance to attacks on the lower, while the compartments themselves are a source rather of strength than weakness; for if one of them, is broken into by a siege engine the damage is localized and its repair made relatively easy. One of the compartments in each tower, that immediately inside the curtain wall and facing the tower opposite, is occupied by a newel stairway.
About AD500 Anastasius, in his efforts to put a further check on attacks on Constantinople, built the “Long Wall,” which, from a point on the Marmora forty miles west of the city, stretched northward across the land to the Black Sea. The wall was about fifty miles in length, and was strengthened by numerous towers. This emperor also fortified the city of Dara, in Asia Minor, on the Persian frontier of the empire.
During the reign of Justinian great strides were made in military architecture. The Byzantine fortifications of this period are among the greatest works of military engineering known. Justinian not only strengthened existing fortresses throughout his vast empire but built numerous new ones. He rebuilt the fortifications of many large cities, strengthened those of others, and built or repaired numerous forts, his military works numbering 700. The building activity of the Eastern empire at this period is without parallel: the fortifications were scientifically and powerfully built, and to them the mediaeval engineers, both Christian and Saracen, owe much of their inspiration.
At Dara, about 140 miles north-west of Mosul, the fortifications of Anastasius, having been constructed hastily, proved to be weak and inadequate. Justinian repaired and strengthened them. He increased the height of the curtain wall by building a vaulted gallery, with loopholes to the field, upon the wall-walk along the whole line of the fortifications, and made another wall-walk with battlements above the gallery, so that there were now two fighting lines, one above the other. A second curtain wall was built outside the first, leaving a space of 50 ft. between the two walls. The inner wall was 30 ft. thick at the base and, diminishing in thickness, rose to the height of 60 ft. The towers in this wall were 100 ft. high. The outer wall was smaller, but was also provided with towers, so placed that they stood opposite to intervals in the inner wall.
One of the towers of the inner wall, called the watch tower, appears to have been of greater importance than the others, and a kind of donjon. This tower was entirely rebuilt by Justinian.
In all his works Justinian gave great attention to the water supply of his fortresses. Dara obtained its supply from a stream which entered and passed out of the city through conduits in the wall, strongly guarded by iron bars. At the inlet the stream was protected by the mountainous nature of the country in this direction; but at the outlet the stream was a source of weakness, since it provided an abundant supply of water to an enemy encamped close to the walls. By the fortunate discovery of an underground passage, which could be entered by a shaft within the city and had an outlet many miles away, it was possible in time of siege to divert the stream into this subterranean passage and so cut off all the enemy’s supply.
At Edessa water was obtained from a river which ran through the city, and which in time of flood caused great loss of life and destruction of buildings. Justinian, by cutting a deep channel through high ground on one bank and building a wall of enormous stones on the other, diverted the main course of the river round the walls. In this manner he not only saved the city from floods, but also provided a kind of moat for the hitherto unprotected walls. The part of the stream still allowed to follow the old course was carried through the city in a stone-lined channel.
At Theodosiopolis the walls were heightened by building a gallery over the existing wallwalk and a second line of battlements above, as was done at Dara. Here the wall-towers were so strengthened that each of them became virtually a keep in itself, and could be held independently.
The above works were on the Persian frontier of the empire. The other frontiers and outposts, from Egypt to the Danube, received equal attention.
Following the conquest of the Vandals by Belisarius, AD533–534, fortifications were built throughout the newly acquired territories in North Africa. Here it was particularly necessary to guard against the forces of revolt within as well as against the activities of enemies without the borders; and the military works were of great variety. They included fortified towns, such as Guelma, Thelepte and Bagai; open towns with fortified citadels, as Haidra, Mdauroch and Timgad; and isolated forts, as Lemsa and Ain Tounga. The first two classes were at once military stations and places of refuge in times of trouble for the civil population of the neighbourhood of the districtor city. The last were isolated castles occupying strategic positions, keeping watch over a plain, commanding an important valley, or guarding a pass. There were also outposts keeping watch at the borders. The Greeks had developed a scientific code of signalling from beacons, by means of which information as to the composition and character as well as of the numbers of an invading force could be signalled from station to station. Scattered throughout Algeria and Tunisia there are large numbers of these Byzantine fortifications, dating principally from the sixth century, many of them still in excellent state of preservation.
These fortresses, though often rectangular, differ greatly in plan, some of them having a very irregular outline. The curtain walls are from 7 ft. 6 in. to 9 ft. thick, and, where the upper parts still exist are from 26 ft. to 32 ft. high. They are often arcaded on the inner face. The wall-walks are protected on the side towards the field by crenellations about 5 ft. high, and on the side towards the bailey by a low wall. They are approached from the courtyard either by flights of stone steps built against the inside face of the wall, or by stairways in the towers. Sometimes greater width is given to the wall-walk by corbelling on the inner side of the wall.
Towers of varying shapes, but most often square, project boldly out from the corners and sides of the curtain. They are usually of two storeys, the basement opening to the courtyard and the upper storey to the wall-walk on the curtain, the division between the storeys being either a stone vault or timber floor. Their battlements are reached by internal stairways. Sometimes the towers have no doorways to the wall-walk on the curtain, and are capable of being held independently.
The gateways are always defended by one and sometimes by two towers, one on either side. Often they pass through the lower storey of a tower, either going straight through or, entering through one of the lateral walls, take a right-angle turn within the tower before passing into the fort.
As at Nicaea and Dara there is often one tower which is larger, stronger and better fortified than the others. It occupies a position either at a strategic point behind and completely isolated from the curtain, or is in line with the curtain at the highest point in the fort or at a point particularly exposed to attack. This tower was in all essentials of its design and purpose the prototype of the rectangular keep or donjon of later times. It was the strongest building in the fortress, was capable of offering independent resistance, and was the place where the last stand was made.
Bagai, Algeria, is a good example of a fortified town. Here three of the wall-towers, projecting boldly out from salient angles of the curtain, are round, while the other towers are square. Occupying a commanding position on the north side of the town, there is a citadel, and within the citadel a powerful donjon about 85 ft. square. Haidra in Tunisia, and Timgad in Algeria, have well preserved citadels, each guarding an open town. In the former the curtain walls are built with internal arcades.
The citadel at Timgad forms a regular rectangle on the plan. There are towers at the angles and in the middle of each side, all of them square and all projecting boldly on the outside of the curtain. The principal gateway passes through a particularly large tower in the middle of the north wall, and was defended by a barbican. The barbican, which is of slightly later date, was entered through a lateral wall. The inner door of the gate was flanked on either side by a tower, now destroyed; and from each of these towers a mural passage runs through a lateral wall of the gate to the outer door, so that an enemy who had carried the first door, and was held up by the second, could be attacked from the rear by men issuing from the inner towers through the passages. In the south wall of the citadel there is a postern defended by the middle tower on that side.
Probably the best preserved of the isolated forts is that of Lemsa, Tunisia, a rectangular castle with corner towers. Three of the walls stand to their full height, from 26 ft. to 32 ft., and retain their crenellations. Ain Tounga, guarding a pass in Tunisia, is of trapezoidal plan, and has a tower at each corner, one of which (a strong rectangular building about 36 ft. by 40 ft.) is much larger than the others, and projects entirely on the outside of the curtain. The gateway passes through a tower near the middle of the south side of the fort, entering at one of the lateral walls and taking a right-angle turn in passing through.
Timgad: Plan of the Citadel
The castle of Gastal, Algeria, forms a single rectangle with a round tower of bold projection at each corner and a square tower in the middle of one side.
In addition to the above types there are also numerous smaller forts scattered throughout Algeria and Tunisia, here guarding a narrow defile, there the approach to a village or important agricultural centre. These forts are plain square or rectangular buildings, having no corner turrets and only one gateway.
The fortifications of the Roman emperors on the Danube consisted mostly of single towers posted along the river, principally on the right bank. These towers, which had been destroyed by Attila in AD446, were rebuilt by Justinian in much stronger form, and many others were added at suitable points.
Plan of Aïn Tounga
At Episcopa, near Silivri on the Sea of Marmora, Justinian built entirely new fortifications, designed by one Theodorus Silentiarius, “a very clever man.” Here the wall-towers were of such bold projection that they commanded every point of the curtain between them. The gates were not designed in the usual manner between two towers in the wall, but each gate was placed at a point where the curtain took a short right-angle break inwards before continuing on the same line. The gateway was at the side, through the short wall of the break, and was therefore hidden from view. An enemy attacking one of the gates found himself in an angle of the fortifications, exposed to fire from the curtain wall on his flank as well as from the gate directly in front of him.
In fortifying the Dardanelles Justinian built a strong fortress at Elaeus near Cape Hellas at the western entrance to the Straits. Here he constructed a wall of great width and height, and dug a deep ditch in front of it. Upon the wall he raised two storeys of battlements, the lower one of which was vaulted and contained chambers for the garrison.
When Belisarius repaired the fortifications of Rome, after his capture of the city in 536, one of the improvements he made was the addition of wing-walls to the merlons of the parapets, like those at Pompeii referred to above. Remains of these merlons are still to be seen. Belisarius also surrounded the wall with a moat.
The rise of Moslem power in the seventh century put the fortifications of the eastern empire to a severe test. Between AD637 and AD655 the Saracens had conquered Syria, Egypt and Persia; their fleets had swept the Levant and taken possession of Cyprus and Rhodes; and in AD668 they appeared before the walls of Constan-tinople and laid siege to the city. The siege lasted until AD675, when the Saracens were beaten off with great loss; but it would appear that the Byzantines owed their preservation as well to their use of that powerful weapon “Greek Fire” as to the strength of their fortifications.
Greek Fire (the precise composition of which is unknown, though sulphur, naphtha and quicklime seem to have been major ingredients) was a terrible weapon both on land and sea. On land it was made up into tubes, phials or pots, and either cast by hand or projected from engines at the end of arrows and bolts. On sea it was blown out of large copper tubes erected on the prows of vessels selected for the purpose. Water would not quench it but rather spread the fire hither and thither.
In AD716 Contantinople was again invested by the Saracens, who attacked the city both by sea and land. The siege lasted thirteen months, and ended in the utter destruction of the Saracen fleet by the fire ships of the emperor and the rout of their army at the land walls of the city by his allies.
From this period to the date of its fall in the fifteenth century the empire was constantly assailed from all quarters; but the traditions of skilful military architecture were well sustained throughout, particularly under the Isaurians and the Comneni.