Helltowers are large towers made of netherbrick which are summoned by the Demons via Chaos Magic. Accompanying the creaton of a Helltower is unusually high amounts of volcanic activity (including eruptions of lava) and the appearance of massive monsters, such as the Tyrannosaurus-Rex encountered by Tycer, Myles and Magmamale at the base of the first discovered Helltower. At the top of every Helltower there is a Nether Crystal, occasionally two, which is the source Helltower's Chaos Magic. Fifty Helltowers are known to exist in the overworld of Mearth, but only two have been discovered - these two were were also subsequently destroyed. The purpose of the Helltowers is currently unknown.
The Demon Fortress in the Nether is comprised of many Helltowers. However, these Helltowers do not have a Nether Crystal on them and therefore appear to have more of a structural purpose over hotspots for Chaos Magic, like their counterparts in the overworld.