As the name suggests, Tower 101 was a 101-storey building. Poor sales left the building vacant above 64th floor. The property agent even decided to seal up the passage going up to 65th floor. In the end, residents could not escape to the rooftop, nor even have time to break through the sealed walls.
Residents living below 65th floor began to set their defensive measures on their own floor. Different floors had different number of people. On some floors, the residents boarded up the elevator entrance and both staircase exits, creating an enclosed environment on the whole floor. On other floors, some could only fortify their own apartment's front doors and walls to protect themselves. Some others took what they needed and grouped up with residents on other floors.
On the 120-minute mark after the outbreak, zombies had occupied the lobby of Tower 101. Zombies could hunt their prey, but certainly did not know how to operate the lift. When there was no prey, they go into a near dormant state and feed on whatever there was.
Zombies are naturally attracted to one another. As they see zombies in front of them, they will follow. As a result, more and more zombies were heading towards Tower 101. As more zombies and blood attracted even more zombies, they managed to spread themselves among 1st to 40th floor. There were basically no survivors remaining below 40th floor - even if there were, they were trapped in their own apartments.