Monster Train... Slay the Spire with... I guess you could call it an extended wave mechanic? I like it... but man, does it have some annoying things.
I just finished my second run and preliminary impressions...
Like the last nth ramble on games of this genre, everything's being compared against StS except when explicitly stated.
So two fundamental differences: You're summoning units that take their actions during the inter-turn phase, and there's 3+1 layers on the battlefield. Each layer has a finite amount of slots for units (both your side and their side), and enemy units will move up a layer after combat is resolved for the turn. If they reach the fourth (+1) layer, you take damage and counter with a heavy hit. After you summon/cast spells, your enemies do their thing, your guys do their thing, the boss (if there is one) does its thing, and new hostiles move in from the first floor.
It feels kind of slow in retrospect. But I'm also accustomed to like 15 minute Heart runs on StS, so it's probably that I'm just used to faster play. Edit: Probably should, ya know, actually look at the settings panel sometime. What I get for not sleeping...
Both runs used very different strategies, owing to different primary/secondary decks. I actually expected to fail my second run quite handily and... instead stomped the boss. (And almost lost to the second boss.)
RNG feels 'weaker' in this game, with more choices in terms of upgrades. Also, your card pool is more limited, due to the primary/secondary clan system which reduces the number of lousy cards you can possibly get. Battles alternate with a dedicated set of bonuses, of which you have to choose from the two sets presented. (Do you want a magic vendor, free trinket, and gold, or hp, unit vendor and free card removal? etc.)
My main issue up front would be... I fought the same final boss on both runs. For some reason, the first time, the boss had about double what it had the second time, for no discernable reason.
Update: So spending some more time with this... one of the major mechanics is coming across as awful. At the "end" of a stage, a (mini)boss pops out and proceeds to lay the beatdown on everybody with autoplayed turns. The practical consideration here is that their side acts, your side acts, end of turn effects proc, repeat. You never have a chance to use your hand again until one side on that floor dies. Oh, you play a spell deck instead of a creature deck? Screw you. But this is an inherent game mechanic so you can pretty much ignore going all-in on an entire set of mechanics.
There's another really stupid mechanic combo that's basically one-step away from an auto-loss: One artifact autosummons a bunch of your units to the second floor for free at the start of combat. Sure, why not-- in return for free summons, you lose the advantage of tactical positioning and maybe the use of your first floor. Oh look-- that miniboss has the Haste passive. Which means it goes from floor 1->3 directly. And because you don't know what combats you'll encounter, you can't work around this one short of not taking that artifact at all.