@RIGHEOUSwench

How many soldiers are in the fort? It fluctuates as recruits come and go, but I'd estimate somewhere between 500 - 700 (which means it is woefully undermanned as it was built to accommodate about 1,200 mean comfortably, 1,500 at a push). Three quarters of these are regular soldiers, the rest are ranger/scout types who patrol the nearby mountain passes. 

There's a couple of reasons why the fort is so poorly maintained:

It's built in a mountain pass, blocking the only entry from the mountains into the empire; the tribes that live in the mountain are a fairly crude people. The last time they were a real threat to the empire was almost a century ago, when they last united under a single leader. After some fierce fighting they were eventually defeated and split apart into a dozen squabbling tribes fighting amongst themselves. No one ever thought they would be united again.

The empire has been weakened by complacency and internal strife: there was a struggle for the leadership between the previous emperor's descendants 15 years ago, and many of the old soldiers in the fort are decent men who happened to be on the wrong side when the fighting was over. As they swear an oath of loyalty to the people of the empire not the emperor they're still loyal men, but their posting to the fort is simply to get them out of the way and let them rot, and they know it - more than one has taken a leap from the battlements before now.

If the soldiers retreated and let the fort be overrun the tribes would have a direct road straight into the wide farmlands of the north - the tribes could spread out and overwhelm the soldiers pretty quick in the wider lands, and it would take time for the empire to get a large enough force together to defeat them - time in which civilians will be slaughtered and their lands razed. The fort in the mountain pass is the only place narrow enough for them to stand a chance of defeating them.

(If you've ever read [i]Legend[/i] by David Gemmell it should give you an idea of what I'm thinking of here.)