This reminds me of an old Counter-Strike: Source gamemode called Buildwars this was a mod that servers were running back in the early 2010's era and possibly before. It was alot of fun, I still have memorys of being a squeeker and having tons of fun with it. I think for the building aspect to really shine the maps need to be big and mostly empty but not bland you know so actual forts can be built and defended. It also reminds me of an old hl2 mod called Source forts.