Tattershall Castle was defended by an incomplete outer moat, which, starting from the north-east angle of the area, went along the north and west sides, and communicated with the river Bain; there is still some water in portions of this ditch. It probably was carried eastwards so as to enclose the church, but there is a noticeable dip in the ground on the eastern side of the castle area, which is probably the same moat defending that portion of the castle. From a plate of the castle published by Buck in 1727 (which is in the writer’s possession), much knowledge can be gathered of the arrangements and buildings of Tattershall at that date. The inner moat was complete, and the outer wall which surrounded it, for the most part of brick, is in fair condition; it was supplied with water from the outer moat by a culvert piercing the outer bank and wall about the middle of the north side, which is plainly visible, and around which the bank is faced with stone. This, according to Buck, was protected by a strong and tall tower over it. On the east side of the moat may be seen a blocked up arch, which was connected with the outer moat at that point, and ran eastwards beneath the church to that part of the moat. The main entrance to the castle was evidently a little north of this arch, much in the line of the present pathway, for Buck plants a large gateway with two towers and portcullis here, on the inner side of the inner moat. Just outside and to the eastwards is a brick building of two storeys, and of exactly similar work to that of the rest of the castle. This house, which was probably used for guard-rooms, had at the north-west angle a turret staircase, and the only door on the ground floor was originally on the southern end, opening on to a terrace wall which defended this part of the castle. The outer wall was about twelve feet high (judging from the mark where it joined the house), with arched recesses and loopholes, and over all a platform with a crenellated parapet. A brick turret was at the south end of it, and the ruins of this and the drain into the moat are still visible. On the south side of the inner moat can be seen some stone corbels in the brick-facing wall, which were evidently to support a drawbridge. On the west side of the outer wall is a large brick one-storeyed building (with little arched cupboards or recesses in the walls, as will be found elsewhere), probably a guard-room, and the ruins of a strong tower farther to the north and east, on the edge of the outer moat.