The ISB contains up to 10 levels that a player can attack. Each level has an increasing level of units - from 60 Black Widows on level 1 to 1,000 each Pultori on level 10. Floors 5, 8 and 10 each has a boss that will fight alongside the units there: Harry 'Pittsburg Phil' Strauss on the 5th, Jackie "Legs" Diamond on the 8th and Albert "The Mad Hatter" Anastasia on the 10th.