Dungeon is an evil alignmenttown. As the manual states, dungeons are built by the warlock and overlordhero classes to act as bases from which to wage campaigns of conquest for wealth and power. Similarly minded creatures are attracted as allies. Other dungeon creatures are in thrall to their masters. Dungeon armies are possessed of a variety of long ranged attacks, have effective damage dealing troops, and have the ability to greatly disrupt the strategies of their enemies. Dungeon represents Nighon.
Or, to not damage ones own units, the Armageddon's Blade (artifact), if tier 7 creatures of Rampart and some other immune units are not part of the opponent or can be killed later.
Although dungeon is quite expensive, it has good magical capabilities (e.g. mana vortex) and powerful troops like black dragons, which are among the strongest creatures in Erathia.
Pros
Two ranged units, both of which have no melee penalty.
Mana Vortex: doubles your max spell points once a week.
Black Dragons are potential for "Dragogeddon" tactic because of their spell immunity. Additionally Warlock gain more power than other hero classes.
Guardian of Earth - (Grail Building) (+12 spell power to defending hero Dungeon special, else 5k Gold each day additionally, +50% creature growth like other Grail buildings)