in east Kent, an anchorage which gave access to Deal Beach and on which an invasion force of enemy soldiers could easily be landed. These defences, known as the castles of the Downs, were supported by a line of four earthwork forts, known as the Great Turf Bulwark, the Little Turf Bulwark, the Great White Bulwark of Clay and the Walmer Bulwark, and a long defensive ditch and bank. The route inland through a gap in the Kentish cliffs was guarded by Sandgate Castle. In many cases temporary bulwarks for artillery batteries were built in during the initial stages of the work, ahead of the main stonework being completed. The Thames estuary leading out of London, through which 80 percent of England's exports passed, was protected with a mutually reinforcing network of blockhouses at Gravesend, Milton, and Higham on the south side of the river, and West Tilbury and East Tilbury on the opposite bank. Camber Castle was built to protect the anchorage outside the ports of Rye and Winchelsea, defences were built in the port of Harwich and three earth bulwarks were built around Dover. Work was also begun on Calshot Castle in Fawley and the blockhouses of East and West Cowes on the Isle of Wight to protect the Solent, which led into the trading port of Southampton. The Portland Roads anchorage in Dorset was protected with new castles at Portland and Sandsfoot, and work began on two blockhouses to protect the Milford Haven Waterway in Pembrokeshire. In 1540 additional work was ordered to defend Cornwall. Carrick Roads was an important anchorage at the mouth of the River Fal and the original plans involved constructing five new fortifications to protect it, although only two castles, Pendennis and St Mawes, were actually built, on opposite sides of the estuary. Work began on further fortifications to protect the Solent in 1541, with the construction of Hurst Castle, overlooking the Needles Passage, and Netley Castle just outside Southampton itself. Following a royal visit to the north of England, the coastal fortifications around the town of Hull were upgraded in 1542 with a castle and two large blockhouses. Further work was carried out in Essex in 1543, with a total of seven fortifications constructed, three in Harwich itself, three protecting the estuary leading to the town, and two protecting the estuary linking into Colchester. St Andrew's Castle was begun to further protect