UnderMaster is described as 'Plumb the depths of the Earth in the spooktacular Browser Game: Under Master. There, you’ll build your own dungeon and expand it using the hideous creatures of the deep'. There are three alternatives to UnderMaster, not only websites but also apps for Linux, Windows, Mac and Steam. The best alternative is War for the Overworld. It's not free, so if you're looking for a free alternative, you could try OpenDungeons.
Tired of invading dungeons? It's time you build your own! War for the Overworld is a Dungeon Management Game that employs your favourite aspects of the RTS and God Game genres. Carve out your Dungeon and conquer the Overworld!. Represent entire series of War for the Overworld games including War for the Overworld, War for the Overworld Heart of Gold expansion.
OpenDungeons is an open source, real time strategy game sharing game elements with the Dungeon Keeper series and Evil Genius. Players build an underground (or overground) dungeon which is inhabited by creatures.