Assault on Tavern 13

I have an idea for an encounter I'd like to run for my players, but need some advice on the best way to make it fun / with the right stakes.

Basic backstory: The players have been investigating who is behind an attack that is being used as a pretext to launch a devastating war. They've recently captured a goon/fixer ("the Duke") who has been pulling the strings on behalf of a powerful merchant who is allied with a group of devils/demons/etc. They captured the Duke pretty far out in the woods, and have a long trip back to civilization where they can hopefully produce the Duke and the papers they found on him as proof of who was really behind the attack, so that the war can be avoided.

So what I have in mind is that they stay the night at a road/riverside tavern, with their captive in tow, and wake up to find the place surrounded by goons of the merchant/devils/etc., who are looking to kill the Duke before he can squeal and mess up the plot to start the war. Basically, Assault on Precinct 13, but it's a tavern instead.

I'm struggling with how to make the combat encounter more than just a slug fest - any ideas?