In chapter 58 of Ghosts of the Past, at the end of the Mirror Image arc, Reynolds a.k.a.

Needless to say, most of the cast is absolutely gobsmacked. Since he'd tweaked it, however, the result is a huge white tower jutting out from the now neutralised volcano, with structures running underground and into the mountainside - it's intentionally designed to serve as a mine/processing point for vibranium and mithril, potentially a powerful defensive structure (considering that it's Made of Indestructium), and above all, as a magical university. Doctor Strange then put the destroyed landscape in a time bubble, where a millennium passed overnight, and used a piece of Atlantean techno-magic that would use the surrounding materials to build an Ancient City-Ship from Stargate Atlantis. It's on an island in the middle of the Hogwarts Lake, after the island was transformed into a volcano by an Earth magic-wielding dragon that was out to turn it into a super-volcano and destory the world, then its insides were transmuted into a mixture of vibranium and mithril ore by Doctor Strange (using the Philosopher's Stone, which he'd nicked) to slow the production of lava (as both materials take a lot more energy to melt).

One appears in chapter 43 of Ghosts of the Past, which as one character puts it, looks like 'a palette swapped Isengard'.

In present day, it's long fallen to ruin.

The Apprentice, the Student, and the Charlatan: Starswirl the Bearded, the greatest wizard to ever live, had one at the very northern edge of Everfree Forest when he took residence.

Mage tower arcane mage