I know what this reminds me of!
It's RAMPART by Atari (yes i talk about Atari a lot!) in 1990 or 1991 or so.
Which has incoming things from the sea and you build individual castle walls but in Rampart you lose if the wall has any openings at all it has to be inner-contained.
Speaking of my music from my comment when you rated -my- game.
"Licence to Crenellate" pertains to Rampart but as a mythical sequel to the game.
Compare Rampart's music on YouTube then listen to this:
https://soundcloud.com/doug-stoothoff/crenelwav
cheers