Walmar Castle is the most southerly of the three Henrian coastal forts which protected the Downs, that sheltered strait lying between the coast and the Goodwin Sands. It stands a mile from Deal Castle, to which it was originally connected by earthworks, and was built at the same time. Though resembling Deal in principle, it is simpler in design. It consists of a squat cylindrical tower closely surrounded by a lower curtain, the latter projecting outwards in four semi-circular lobes to form a quatrefoil plan. It was a plan shared by Sandown Castle, the northern member of the group and now almost totally destroyed.
Walmer Castle stands in its entirety but, in contrast to Deal and most of the other Henrician forts, it austerity has been mellowed by conversion into a stately home. In 1708, the militarily redundant castle became the official residence of the Lord Warden of the Cinque Ports, a medieval office which has survived to the present day as an honorable sinecure.
The transformation to a mansion is all the more remarkable given that the low, curved, immensely thick walls cannot have lent themselves easily to such a purpose. Fine gardens now surround the castle and encroach upon its deep, stone-faced ditch, while many of the gun embrasures have been converted into windows.
When first built, Walmer exhibited the usual Henrician defensive arrangements. Cannon would have been mounted on the parapets of the central tower and outer curtain, a third tier of fire being provided at the level of the ditch by gun ports in the curtain. These gun ports are linked, as at Deal, by a continuous fighting gallery in the thickness of the wall. The central tower provided the main accommodation for governor and garrison. The lobe containing the entrance was heightened in the 1860s to provide further accommodation.