House of Harpies (3e)
The local thieves' guild survives by hiding outside of town¸ in the nearby forest. Since the thieves confine their predatory efforts to within the city¸ local druids and rangers have no reason to look for them¸ and the town guard concentrates its efforts within the town's walls. Over the past several decades¸ the guild managed to build a well-concealed hideout in one of the large evergreen trees near the town. This allows the thieves to plan and plot in safety¸ gives them a place to rest and hide if things get too hot¸ and lets them check caravans coming into the city for likely targets.
"Unfortunately for the guild¸ their ideal hideout attracted a flight of harpies that moved into the area. The harpies like the hideout for many of the same reasons the thieves do¸ and have driven the thieves out and taken up residence. The thieves managed to escape with most of their secret documents and important possessions¸ but now lack a good¸ secure base of operations. The leader of the harpies¸ a particularly nasty creature named Irith¸ hopes to use the treehouse base as a headquarters from which she will expand her flight¸ becoming a force in the area with which to be reckoned.