@ShionSinX
That would be a poor use of resources. I found her annoying too but now makes sense seeing who niece she is.
The hero is leaving and he noticed him, can't surprise attack someone looking dead at you. Plus he's recovering from using all those high aoe healing spells, even if he's weaker that could end badly for MC. That's poor planning and power of observation. Not to mention boring conclusion for a story.
You would have to evacuate the civilians which there most certainly would be in the place first (and yes this is important since you're trying to liberate the citizens from this army). That would seem suspect to any soldiers guarding the place. But if you'll able to do that, it's a pretty solid idea. But you'll want to refrain from killing all, never underestimate the value of information. AND you'll need someone to intercept any messengers and what not. Unless the army is completely incompetent, there's no way they would send 100% of the soldiers no matter what happens. Others would leave to inform the main army and surrounding platoons.