CRYPT v. BRAMBLE:

Bramble has more Gremlins & Lucky Wraiths it looks like, but also those Cat Burglars who can repeatedly steal or 3x hit. It also introduces enemies who Rage-stack. Compared to Crypt where the bulk of the gameplay there comes from larger packs of mobs and ratwolves who alternate attack & defense.

DEEP CAVE v. SWAMP

Deep Cave has the Fox Warrior miniboss in it that sometimes shows up in Events. He gives the Fox Rapier - one of the best vanilla weapons in the game - but is pretty damn tough to beat without proper prep (you need at least 30 block per turn and/or either good DoT or good burst damage - at least 40 points or more - to handle him consistently). Beyond that, Deep Dave has the Rabbits who can hit for 20 points every other turn and the Badgers which signal Badger Boss. Swamp, however, is all about Thorns & Poison. You're dealing with Newts who give spikes every other turn, Salamanders who poison every 3 turns, and frog sages who poison every other turn. Swamp also has the Frog Bath event that can either be a ton of experience or a Max HP buff.

CORE v. HEART

Magma Core is all about Burn and the Olms/Cat-Captains. Olms & Cat Captains get Rage basically every turn, while any mob in black robes will hit you with Burn pretty consistently. You need high block or really good single-target damage to survive consistently here. Frozen Heart, however, is mostly standard save for one of the hardest regular encounters in the game: the Enlightened Cultist Six Pack. Here you need REALLY good AoE damage or a way to make yourself functionally immortal to consistently survive it.

For each one you also get 2-3 different boss options depending upon your run seed. Crypt has Rude Rooster, Ultra Knight, Ferret King, and Queen Bee. Bramble has Hawk Knight, Ultra Knight, Bandit Captain, Queen Bee, and Bandit Lord. Deep Cave has either Badger Boss or Blademaster. Swamp has either the Speaker or the Elder Mage. Core is King or Grandmaster. Heart is Dark Lady or Farseer.