Ok, so I'm ready to start this all over again, but let's reiterate. You can redo your spells since you just rested. You are going after an imposing fortress, trying to find a way in.
The description was as such: "Twenty feet tall and flanked by a pair of gangly stick-towers, the gatehouse stands resolutely in the mist. Snarling figures stalk the parapets, before flaming braziers belching clouds of thick black smoke. Smaller figures crouch atop the towers, peering warily through the dim light."
There are towers lining the walls, separated enough to be able to take out one, but not separated enough to sneak over the wall without easily being seen. Several kobold troopers man each tower.