Or do I just suck?

I've been playing for a bit now and I keep losing when attacking small towns (the ones with the three ways in and the towers) even when vastly outnumbering the defenders, with units of better quality and experience. It just seems the second that a unit is bogged down in one of the corridors, no matter what unit, or what's facing... it's basically done for.

Am I missing something really fundamentally obvious?

I've tried throwing everything at two corridors and then using the third to sneak round back and I'm still not getting anywhere.