Hear this sad tale for an example of bad luck and stupidity using forts :
I had a stack of all elite heavy troops in recently conquered land, somewhere in the baltic region. I think "hmm, a watchtower will be nice over there" and send my general alone to build it (moves faster without inf). The target position was some 2 turns away. I figured I could use some anti-desertion insurance so I built a fort before the gen left the stack. One or two turns later, I get attacked. And not just by any army - you know those most useless, clueless stacks-o-trebuchets-with-militia-general the AI likes to build ? It was one of those.
Almost half of my silver & gold shiny knights died a fiery death inside that fort.