I had a group of 4 pcs run this encounter.twice. first time they found out about the lamia and waltzed right into the tower, got to the top and found no lamia (since she was invisible). Then due to some poor will saves against the major image the barbarian died and got thrown off of the tower and the warpriest got skewered through the eye a couple times (he died too). Anyways the pcs got away only to return to kill the stinking bad guy who had killed their friends. Long story short they got the tower dropped on their heads which almost killed the ranger and smooshed his pegasus to goo. They did manage to kill the Lamia with a lucky greater slaying arrow from the ranger. however it was very close and very memorable.
If you want to make the encounter seem extreme just drop the tower on their faces and watch the encounter of pain unfold.