In many Chinese people's minds, 塔 are buildings with triangle shape, and 楼 are more flat, which is usually true.
While, if you are into the formal differences in terms of architecture. The difference between 塔 and 楼 is that 塔 has Sōrin (which is the vertical shaft which tops a Japanese pagoda, whether made of stone or wood) while 楼 does not. Certain 楼 may have a triangle shape but does not have Sōrin is called 楼, such as 黄鹤楼. And any architecture with Sōrin is called 塔，even it has only one floor, such as 山东历城神通寺四门塔。
Please note that this definition may only suit traditional Chinese architectures, and may not necessary fit our naming for modern architecture such as (Eiffel Tower).