What floor is my empty island built on?

My empty world island was built on the 1 1 floor. The height of the empty world island that I can fall to death is 1 1. It is the best choice to make 1 1~ 15 when building strange towers. Just in case, you can dig out the square facing zombies under the tower to prevent monsters such as zombies from coming out. Some monsters may wear armor or something. It can also help to absorb experience and acquire weapons.