I believe that they will leave fortifications if they are set to be aggressive, if an enemy comes within charge range, or possibly if an archer is shooting at them and there are no other units on the field.
The archer thing I'm unsure of; while it would be logical for them to charge, and I believe I've seen them do so before, I do with utmost certainty recall a situation 5+ years back in which an army made up entirely of archers attacked a city. The aggressor's archers made short work of the enemy's archers (which were minimal or possibly non-existent) leaving only infantry defending the wall. The infantry, rather than seeing the futility of what they were doing, elected to stand there doing nothing as they suffered volley after volley of enemy fire. The battle ended in a stalemate, as the aggressor's forces, although unimpeded, weren't potent enough to kill all of the enemy infantry within the time frame (25 turns I believe) that a battle takes place in.
The aggressor was Sirion and the defender, I believe, was Oligarch. The city in question was Ashmoor.
