The ground floor, first floor and roof of the building are inaccessible. The second floor is accessible from the ground via some metal wreckage and is connected to the small section of the city sewers nearby, which leads to the third floor of the building. The layout of both accessible floors is relatively simple, since there are almost no inner walls left.
Part 2 of the Nostalgia challenge is located on the second floor on the "Office of the Treasurer Terminal" (Unusually, this may not add a note to your PipBoy, although reading the terminal still counts toward the achievement.).
There may or may not be two deathclaws outside this building. Their appearance depends on whether or not marked men in the building just northwest upon entering the area after blowing up the bomb are killed.