[Taqwus]

Very situational.

An army inside a fortress -- especially a flying army -- threatens lines of communication of anybody who attempts to bypass it. Whether or not this is a problem depends on whether the invaders have enough supply items, have another way to bring in reinforcements as needed, and whether there's a risk of being wiped out wholesale due to fleeing while surrounded (ex. by a surprise Wailing Winds and other heavy fear attacks).

Fortresses used to be much more of a no-brainer, 'tho; refer to the D2 "mad castling" approach of taking cheap fortresses esp. Watchtowers and spamming them, partly because of the availability of Ghost Riders (which is now D6 instead of, er, D4 if memory serves). The changes make it both far more difficult and less productive to do this.