The Eastside Mob (based on their appearance) are an organized crime syndicate on the Third and Second Island of Paper Thin City. Their HQ is located on the 13th floor of the Mobster Tower on the Second Island which becomes accessible upon killing the leader and his crew, who grant you the “Mobster Tower Key” upon death.
Entering the tower will automatically force you to fight a large amount of gangsters in what looks like a hotel or apartment building. Upon killing all gangsters the reward is an AK-47 assault rifle and $350. The player then cannot enter the tower for seven days, and the difficultly level will increase the next time it is entered.
There may possibly be some sort of rivalry between the Westside and Eastside gangs, judging from their names.
How to get the key
First, you must have many strength points in your stats, so you can fight the Eastside mobsters. They are located in front of the Steel and Hawk/ Vinnie's building. There are three mobsters waiting. While in the fight, they will attack you with a pistol, a double-barrel shotgun and a katana. After you beat them, you will get Mobster Tower Key.