The forts give you
no bonus G. Lemme tell you, on a large map, you better plan you purchases
way ahead or you're a dead man when you head back to the castle with little-to-no G in hand.
This one little thing still jacks me all up when I play the PS1 game and forget about it.
Moral to the story: Pack more G spells. A
lot more.
Fortunately, they added fort bonuses to the DS version, which is based on Culdcept Expansion.