[Cameronius]

Here is Pyros' method for making tall buildings. He showed me this when I was helping him with his ANZAC Campaign. Attached is a map download with an example.
1- Choose your building size and location.
2- Use desired terrain height(100 in my example) to cover area building will occupy. Slope hexes should be covered by building.
3- Cover heightened terrain with impassable terrain.
4- Place building in desired location.

In my example the outside edges of the building are at height 95, inner hexes of the building are 88. Hexes immediately surrounding are at 0.

This technique provides you with a tall and impassable building with no surrounding slope.

Hope you find this helpful.