Take me down… deep underground… “Dungeon crawler” is something of an umbrella term because it encompasses a wide variety of RPGs. The basic notion is that your characters are crawling around a dungeon rather than in an open, surface world, generally with lots of different levels of dungeon. But within that, it can be a conventional RPG, a rogue-like, 3D or 2D, randomly generated or pre-designed, and it may or may not feature traps, tricks, puzzles etc.
Many non-dungeon-crawler RPGs can also include extensive dungeons that need to be crawled through, so the boundaries can get blurred. Some are fun, some are hardcore with no minimap and you’ll be resorting to pen-and-paper to keep track. The good thing about dungeon crawlers is that most have minimal (if any) IAP.
Doom II is one of the classics. Originally released in 1994, thanks to the release of the source code, it has been updated and ported across numerous platforms in the decades since. It has turn-based combat and includes puzzles. This is also how an FPS (first person shooter) originally looked!
Moonshades and Legend of Grimrock are two 3D dungeon crawlers that might appear old-school turn-based from their graphics, but are in fact both action RPGs. Moonshades has various special dungeons and pits that you can access to gain extra loot (such as legendary level items) separate from the main dungeon levels.
The Enchanted Cave 2 offers 100 levels of retro-graphic random dungeon diving, tonnes of loot and crafting. It’s a roguelike but the penalties for dying aren’t too dire – you only go back about ten levels, and you hold onto legendary loot. If you prefer your dungeons pre-designed (making it easier to find guides, hints and maps online) try Mazes of Karradash 2 for a retro look with a tonne of well-crafted puzzles.
The Adventure to Fate dungeon crawlers, available as a bundle, offer a more JRPG-feel alongside the dungeon crawl aspects. Another very full-featured game is Lootbox RPG, which includes features such as crafting, a spelling to summon a merchant, a mining skill that enables you to harvest resources, and Zombie-mode for automatic dungeon grinding.
These and others can be found in the Dungeon category on this site.