The AI in DLV attacks forts...If you make permanent ones on the campaign map.... It will only take them if it thinks it knows it can win.

A funny side effect. It improves AI pathfinding so turn times are shorter

The caveat is they are having trouble getting the AI to sally out when besieged if outnumbered... It just lets you starve them to death...

One answer could be to have additional troops spawn in the forts every ten turns or so...

There is no reason the forts have to be ahistorical... With those tools you could make any type of forts you want... Even old school Gaulish/ Britons raised earth forts...