I'm looking to see if the following statement holds water. Please challenge, question or add to the idea.
Forts
Forts can be traps. If you don’t have a field force capable of relieving a threated fortress, or if delaying the enemy is not critical, fight your defending forces and garrison outside the structure in entrenchments.
Outside, if the defenders survive the initial combat round, they may be able to retreat. This might limit NM loses you would suffer if your force was trapped inside and fell to an assault or siege. In the same spirit, if your fortress artillery might be captured and of use to the enemy, eat the victory point cost and spike the slow moving guns.