Martello towers were built on the southern coasts of England between 1796-1812, to deter a French invasion. Blake and his wife lived further west along the coast, at Felpham, between 1800-3. He may have conceived the words which we now know as Jerusalem there.
Blake often called the island of Britain ‘Albion’. Being so close to France, its south coast was militarily very active, as the soldiers seen here suggest. While at Felpham Blake was arrested as a spy, probably because his activity as a miniature painter was mis-heard as ‘military painter’. Jerusalem is not, of course, about military strength.