OOC II
We don't really know much about it either, but I suspect it is well-defended. The 100 foot clearing all around the keep makes me especially nervous. It seems highly unlikely that we'll be able to make it to the keep unnoticed given the amount of open space we'll need to cover.
Lathan does have a longbow and the guard should be in its non-Disadvantage range. A distraction from one of the familiars could give Advantage to make the attack more likely to hit. Lucien, you should be proficient in longbows, right? I would gladly lend it to you; I think your ranged attack bonus is better than mine anyway, and you could potentially benefit from a sneak attack. Of course, if that doesn't take him out, he'll surely alert anyone else inside to our presence.
Also, how are we going to get inside if we do make it to the keep? The gates are barred from the inside, so I doubt we'll be able to force our way in. The walls are 15 feet high, so we could potentially scale them with rope. That's probably our best bet.
A lot to think about. Whatever we decide, Lathan will be there, but we need to be careful.