Seventeen feet within this outer wall the basin is crossed by an arch, supporting a light wall of brick and timber, which was the rear wall of the tower. This arch is a very remarkable piece of construction. It springs from two half-octagonal piers, and is segmental, of 61 feet span and 15½ feet rise. The voussoirs form two ribs, and the inner one is composed of seventy-five stones, united by a simple joggle or rebate. The rectangular tower, which thus rises from the walls of the outer half of the basin, is 86 feet east and west by 18 feet north and south, and capped at its two Thames-ward angles by light cylindrical turrets, three-quarters engaged, and rising above the parapet of the tower. The side walls, prolonged backwards an additional 30 feet, terminate in two square turrets, which occupy the northern angles, and also rise above the battlements.
Two doors on the north faces of these turrets, on the ground level, open into a mural gallery in each wall, looped on one face towards the ditch and on the other towards the basin. These galleries communicate with the southern turrets, and are continued within the south wall, having each two loops towards the Thames and two towards the basin. They do not meet, being stopped by the jambs of the sluice-gate. They were also used to receive the slack of the chain when drawn up.
The floor of the tower, which is above the level of these galleries, contained the machinery for lifting the sluice, which must have been heavy. The walls of this floor are also pierced with loops, and contain two garderobes. In the river-front are two early Decorated windows, of two lights each. The two upper chambers in the south turrets open into this floor, which is reached by a winding stair in the north-east turret, and by an exterior stone stair in the north-west turret.
ST. THOMAS’S TOWER.—PISCINA.
The four chambers in the two cylindrical turrets are of excellent design and delicate workmanship, as may be seen from the plate in the “Vetusta Monumenta.” They are in plan octagons, having slender columns, with high bases and bell caps, all cylindrical, and slightly engaged, in each angle, from each of which springs a light chamfered rib, meeting in a plain joint at the centre. There are thus eight cells, each with a lancet gable, supported by two half-ribs. Three of the faces have windows or loops, and another is occupied by the door. In the lower rooms the loops have chamfered recesses beneath drop arches. In the upper, the loops are larger, but still square-headed, and their recesses have an arch-rib with a hollow chamfer. The south-east upper chamber was an oratory. The window-sills on each side the east windows are Purbeck slabs, hollowed into bowls,—on the south for a piscina, on the north for holy water. Both slabs projected, and have been broken off when the walls were wainscoted.
There is a second floor, reached by a well-stair in the square turrets, which ascend further to the roof, but it contains nothing of interest. The well-stair in the north-east turret leads to a door that opens on the north face, 20 feet from the ground, and which opened outwards, and was barred on the outside. The meaning of this is only explicable on the supposition that a bridge, or perhaps an embattled cross-wall, connected this door with the corresponding opening in Wakefield Tower, 18 feet distant. By this means a person leaving St. Thomas’s Tower, and barring the door behind him, would reach Wakefield Tower, and therefore the palace, and cut off pursuit. In the second floor of St. Thomas’s Tower is another door, above and similar to the first, which in like manner communicated with the top floor of Wakefield Tower; so that either there were two drawbridges, or a wall pierced by a mural gallery at 20 feet high, and with a rampart walk at its summit.
St. Thomas’s Tower was, until lately, occupied by a water-engine, to the great injury of its walls. The upper rooms were cut up into lodgings by means of wainscot and lath and plaster. All has lately been cleared out, and the tower restored in good taste.
St. Thomas’s Tower is attributed, and no doubt justly, to Henry III.; but, although the octagon chambers have an early English aspect, the grand arch, the staircase doors, and the windows towards the river are decidedly Decorated, though probably early in that style. If, therefore, this tower be of one date, it must be very late in the reign of Henry, but more probably it was completed and the grand arch turned in that of his son. The material is a rag-stone laid in uncoursed rubble masonry, like Beauchamp and Salt Towers, with ashlar dressings. The pool below was extensively repaired by Henry VIII. There is some reason to suppose that this was the tower that fell twice while being built.
The curtain along this front is original, though capped and patched in modern times. It seems to have been about 20 feet high above the water, and from 12 feet to 14 feet on the inner side. In parts it was 12 feet thick; but the addition of brick casements in Tudor times, to enable the ramparts to carry cannon, prevents an accurate examination. It is no doubt the work of Henry III. From St. Thomas’s to Byward Tower is 160 feet.