What is the main physical setting in the second half of the novel, The Duplicate?

The second half of the novel takes place in a deserted World War II watchtower on a remote part of the beach. The tower, a "four-story-high square structure, bleak and unadorned," was used by the army as a lookout for enemy boats and planes, and its interior is dark, shadowy, and menacingly prison-like. Its amenities consist of only a couple of rusted metal beds and a door with a rusted padlock, and it appears prominently in local folklore as the site of a boy's murder. This gothic structure serves as an appropriate setting for David's and Angela's battles with his duplicates.