From Ars Magica
Mage Towers are structures that spawn randomly throughout the world when playing Ars Magica. They all contain at least one chest, which will have a random selection of Essences, Spell Recipes and rarely an Affinity Tome.
You cannot place or destroy blocks inside of structures. The only blocks that can be destroyed are leaves,cobwebs, grass, and flowers. To place or break blocks, you must find the Tower Protector. In some structures their may not be a Tower Protector. If there is a Tower Protector, it will be on the inside of the structures.
With the Tower Protector, getting inside of a structure may prove to be difficult. In every structure there will be an entrance. Some entrances are very hidden and other entrances are not hidden at all.
Some of the towers may also contain traps, and these traps only appear some of the time, and not all will appear in every tower. So you need to keep your eyes open when exploring.
Certain towers are unique and are spawned on world creation, or on first load of the mod. An example of this is the Archmage Tower.
Some towers are also guarded by rare mobs, which are difficult to defeat. However, defeating these mobs grants powerful rare items, so it's worth getting the equipment, spells, and enchantments to do so.
Mage Towers will spawn, on average, once per 500 block cube.
A variant of mage towers that spawn underground, mage archives are troves of lost knowledge from ancient times. They contain valuable loot, similar to mage towers, and have secrets of their own...
Mage Archives will spawn, on average, once per 75 block cube. (This needs confirmation as it appears to be much higher than 75)