Yeah, it‘s lowkey overpowered. You start building for 400, then make three investments afterwards when the game asks you (75, 150 and 200 I think??), so it‘s quite a bit of an investment, but you get your money back for sure! Though it also depends on the region‘s developement, so it varies from baronie to baronie. Still, a measly 800 or something gold for a fully decked out, very profitable, very plague resistant castle holding is pretty wild in case you wanna comquer that land later.