Easiest that way to get them to carry their own weight, and the weight of the roof.
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The walls of most buildings are perpendicular to the ground they are built on.
Vertical walls are important to ensure that buildings - particularly older ones - stay standing up. And a wall that is vertical is at right angles to the floor. Having said that, my children went to a school where the floorplan of each classroom was hexagonal. Furnishing it was a nightmare but it is an innovative design. An igloo probably has no right angles but most other building do.
Such a shape is impossible - in 2 dimensions. But a tetrahedron (4 sided polyhedron) can. To imagine one, look up at the corner of a room, where 2 walls meet the ceiling. There are 3 right angles there. Now measure out a distance from the corner along each edge and put a dot there, then just connect the dots, with the fourth dot being the corner, and you have a tetrahedron with 3 right angles.
Get the Walls of Jerusalem built after the Captivity
Form right angles