(In a Bharat game, with a minor Thibetan (Tirut ?) covering, well, Tibet. Four fortress, that keep getting deleted when I do not look at it.
I think if you build through subject interaction it makes the overlord pay but if you do it directly it makes the vassal pay....Fortifications built in vassals lands is not on you ?
(That's a real issue with a game as patched as EU IV, I genuinely think, since nine, ten, eleven expansion ago that fortress maintenance was on the overlord dime)