Comment by YesBox

Thanks! The vertical axis is also broken down into a grid in a sense. Each "cell" is placed in a specific floor as far as the engine is concerned.

So like (C++):

   vector<uo_map<int, ObjStruct>>

Each vector element is a floor. You just need to move the sprite up in screen space to position it.

Though I wish I went with a vector<vector<vector<ObjStruct>>> approach now a days <chunk_id>, <floor>, <object>