Mobster Tower is a Combat Zone building located in the Second Island of Paper Thin City. It can be accessed by obtaining the Mobster Tower Key. It is one of the Combat Zones you can fight in, the others being Gangster HQ, the Sewers, The Lab, and The Brand New Abandoned Warehouse.
The Mobster Tower Key is required to access the Mobster Tower. To obtain the key, you must battle the group of Eastside Mobsters seen hanging out near Vinnie's in the 3rd Island. Before you could battle them, you must have a certain amount of strength points. Winning the battle will give you the Mobster Tower Key.
The Mobster Tower from the inside looks like a floor in a hotel.