Cladun X3

Cladun X3 is the kind of game that hides its complexity and depth of content behind a deceptively straightforward concept. Run through dungeons, beat up monsters, reach the exit. It's basically Gauntlet right up until the customization and character-growth options kick in, at which point it becomes Gauntlet with a PhD. What makes it work is the complexity of each run, balancing beating up monsters, figuring out the trap layout to catch them in the crossfire, looking for secrets in the hand-built levels or attacking a random dungeon to see how much loot you can bring back from its depths, etc. The game is huge and the grind can go on as long as you'd like it to, especially seeing as you can make your own maps and then upload them for others to play. There's a ton of great dungeon-crawling in Cladun X3's depths, with endless tweaks to the dungeon themes and ways to build characters to keep the repetition at bay.