The roof can be accessed via a fire escape ladder that hangs from the Northern half of the back of the building. This last segment of this ladder is almost always secured, but motivated or tall students can still grab a hold and pull themselves up. 

And of course, you might get lucky and find the ladder hanging down, as shown in the picture.

​Park at Piggly Wiggly (6111 W Mequon Rd #2) or Central Bark and walk across the train tracks pictured, over the fence, and into the site, 43.218846, -87.986082. Climb over the pile of rocks to get to the rest of the site. It's a pretty big area to explore. The tower that you can climb should be obvious when you get there. Pictures below will help you identify the climbable structures.