Like in previous games, the Mage Guild in Heroes of Might and Magic IV provides heroes with spells. It can be built in five levels, and higher level towers provide heroes with stronger spells. Unlike in previous games, it's not neccessary buy spellbooks here, since all heroes start with one by default. And the cost is not cumulative - each level costs the same as the one before, apart from second level, which is cheaper than first. The Stronghold does not have a mage guild, as there is no might-aligned magic school.