Odd structures on new pieces of land

I was quite surprised when I saw those images because to my knowledge, Cities Skylines is not a game about defending the motherland and repelling invading armies. Those structures are a system of defensive bunkers and trenches not too unlike the ones seen on the beaches of Normandy during D-Day. Here's a picture of an actual bunker from the side.

I couldn't find an overhead so instead I also included a picture from Battlefield 1942.

This system is noticeably smaller but I believe it shows the point.

However, while you are prepared to fight and die to hold the line now, you won't be able to actually use these structures for anything due to the lack of invading enemies. They are purely decorational in the game.