I think there are several gun-emplacements along the Southern coast of England, constructed at the time mentioned. While the writer was primarily seeking rhymes for "again", there does seem, in the stanza concerning the Nile & Waterloo, an echo of "MacPherson's Fareweel";

"What is Death but parting breath?
On many a bluidy plain
I've braved his face, and in this place,
I'll brave him once again"