[Taqwus]

Fort placement is a bit of an art, as there are multiple possible roles and they have substantial cost in both money and time.

From a military perspective, they offer more rapid troop recruitment of local troops if neighboring provinces have high resources. They also provide the ability to recruit national troops. Both of these are especially useful if you have slow armies and this is near an active theater. Also useful for many armies, they project supply to nearby provinces... another reason for them to be near a front, so you can maintain armies.

Also useful, they may be used for defending chokepoints where you would be able to threaten an invader's lines of retreat and supply... or where your own lines would be threatened if these were held by an enemy. They're also useful for protecting temples (400 gold each!) or critical magic sites. An unprotected temple is a tempting target for raiders, including the assorted remote summons.

A fortress can also buy you a turn of protection for weak forces (a mass of sages researching because you have a Library and didn't bother redistributing them more evenly, say) that may help if you have strong mobile forces (flyers, army movement spells) or magical counterattack (remote summons for stomping raiders).

If you're the Ashen Empire (Late Ermor), fortresses will also affect what autospawns you get.