Only if you view fort spamming as a problem. Is it, if everyone can do it ? Or just a change in the overall strategic shape of the game ?
Well, why not just give every tile an automatic defence bonus and be done with it? This would basically mean that in every military action in the game, apart from naval, the defender would have a big advantage.
And it probably wouldn't look very pretty.

I suspect balancing it so you can afford a few but not huge numbers would be doable.
Agreed