((Getting to the belltower unseen seems a little unrealistic, no matter how you slice it. Sniping the guy seems very unlikely because it's almost guaranteed to not kill him in one hit; flying/levitating over there isn't in the cards. The only outside possibility for that is to get an invisible guy inside, have him/her enter the building, and make his/her way all the way to the bell from the inside without being detected, but even then, the moment you attack the guy, he's ringing the bell.
That basically leaves us with three options. Either Draco goes in and creates some kind of distraction to let us slip in much easier (a fire probably wouldn't do the job since there are multiple buildings in the compound), or four of us go invisible and get in as far as we can before starting a battle on our terms (and leaving one of us plus the animal companions out of the equation), or we just go in with guns blazing and not care that they see us coming.
Resting for the night and doing this the next day opens up additional possibilities. Not only will Draco (I'm guessing) be able to brew up some very useful potions, like Spider Climb and such, but I would also be able to Wildshape into a form that could potentially let me get into the compound without being suspicious. Either a bird so I could just fly in, or if we could see what kind of dogs they have in there then I could just turn into one of those. Also, the Drow won't have any kind of visual advantage during the day, so we'd be on more equal footing.
The disadvantage of that plan is staying overnight in hostile territory, alongside a tied-up prisoner who is certainly a liability.
KM, can we see any dogs from our current position?))
((Edit: Ninja'd by Jackben! Also seems like a good plan.))
Alf nods. He is somewhat mesmerized by the magical moving X's in the dirt, but he's listening all the same. "I am ready."