In Populous, except for the units and some scenery (like grass, trees, eagles, etc), the objects have infinite height.

This is important to know because it makes the game very particular in some aspects, and using the objects height in your favour will help you in many situations. So, this means you have to go around the objects and never across them.

This happens with all buildings and every stone head type. Let's understand this better: if you are on a balloon, you can not fly above any of these objects - you have to go around. If you get blasted against one of those objects, in situations where normally you would fly above them, you will not - instead, you will impact the object, like if that building or stone head had infinite height. 

Experienced players have this memorized, and will use it against you. In online gaming, use spells like blast or lightning according to this knowledge.