As a Russian turtler player I find myself using quite a few forts. And, despite the unpopularity of it.. I have started putting armies in the wooden boxes. The reason for this came because I needed a way to defeat thosse horrid Mongols.

I find that although they do hinder your ranged infantry's accuracy they offer four walls of protection that hinders the enemy even more. Also if you have a general to spare and think he can actually win try putting him in and sallying at night. Since archers can use fire arrows they technically have more accuracy at night and the enemy horse archers are basically firing blind. In one battle I killed two Mongol stacks and lost a grand total of 10 archers using them like this.

If they actually attack you have a advantage for your Infantry since they can wait behind walls until the enemy attacks and there's like a ft or less of closing distance.

I am aware the fort's not the best defensive structure. But my forts in Ryazan are the best defensive structures I own now. But if I hadn't lost it I wouldn't have found a use for forts. Which is basically just a place to sit until the enemy attack.