3. In the bottom of these Marshes, and close to the edge of the river, stands the strong fortress of Tilbury, called Tilbury Fort, which may justly be looked upon as the key of the River Thames, and consequently the key of the City of London. It is a regular fortification. The design of it was a pentagon, but the water bastion, as it would have been called, was never built. The plan was laid out by Sir Martin Beckman, chief engineer to King Charles II., who also designed the works at Sheerness. The esplanade of the fort is very large, and the bastions the largest of any in England, the foundation is laid so deep, and piles under that, driven down two an end of one another, so far, till they were assured they were below the channel of the river, and that the piles, which were shed with iron, entered into the solid chalk rock adjoining to, or reaching from, the chalk hills on the other side. These bastions settled considerably at first, as did also part of the curtain, the great quantity of earth that was brought to fill them up, necessarily, requiring to be made solid by time; but they are now firm as the rocks of chalk which they came from, and the filling up one of these bastions, as I have been told by good hands, cost the Government £6,000, being filled with chalk rubbish fetched from the chalk pits at Northfleet, just above Gravesend.
The work to the land side is complete; the bastions are faced with brick. There is a double ditch, or moat, the innermost part of which is 180 feet broad; there is a good counterscarp, and a covered way marked out with ravelins and tenailles, but they are not raised a second time after their first settling.
On the land side there are also two small redoubts of brick, but of very little strength, for the chief strength of this fort on the land side consists in this, that they are able to lay the whole level under water, and so to make it impossible for an enemy to make any approaches to the fort that way.
On the side next the river there is a very strong curtain, with a noble gate called the Water Gate in the middle, and the ditch is palisadoed. At the place where the water bastion was designed to be built, and which by the plan should run wholly out into the river, so to flank the two curtains of each side; I say, in the place where it should have been, stands a high tower, which they tell us was built in Queen Elizabeth’s time, and was called the Block House; the side next the water is vacant.