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Let's see a couple things.
I don't know what equipment everyone is carrying, but with a rope and grappling hook just about anyone could climb by taking 10.
Also I think the lookout is at street level, since he's in an alcove near the door so I imagine climbing isn't necessary to reach him, if you all want to visit some violence upon him. Or I could be wrong and he could be above the door.
I don't know, but he might be hard to sneak up on if he's situated in an alcove, but depending on how far away he is some ranged attacks might do the trick.
Looks like ranged attacks and reach weapons are a must.
Of course the more actions it takes to take him down, the more likely he'll alert his compatriots inside.
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