A mage returns home after 1 year away and finds that someone has taken over his tower in the city. He wants it back and hires the PCs to reclaim it. He can supply maps etc of what it was like when he owned it (but someone may have moved "Walls of Stone" and placed whole new trap areas etc). The PC's can keep anything in the tower which is not specifically his (of course he can claim anything interesting and they won't know) and a cash reward. No-one knows who has it but he suspects someone respected in the community, hence the attack must be done fairly quietly so as not to warn the current possessor (the mage can prove that he is the owner however, he is not setting them up - unless you want this to happen). The tower is appropriately trapped and guarded, mostly with the expectation of killing the mage who owns it when he tries to return. The guards and traps are there to kill (not capture) anyone breaking in. City guards etc will not take sides unless the conflict ends up outside the tower.