Dwarven Concrete

Dwarven Concrete is one of these hardest known manufacturing material made in the world. While different clans may have their own mix for a given purpose it is a very closely held secret. The dwarves have built bridges, walls, arches, vaults, domes, and foundations from their concrete.

What is almost universally true is the ability of dwarven concrete to resit magic. In addition to whatever other properties the dwarves use in their mix many structures are built from a lattice of iron bars. These iron bars not only provide additional support they hamper any attempt at using magic to destroy the structure. It is not proof against all magic, but it is one of the most resistant materials for its weight and cost.