Slay the Spire is a deck-building roguelike card game in the style of a turn-based dungeon-crawler, in which you choose one of four characters and try to ascend through a series of increasingly difficult combat encounters and random events so you can defeat the heart at the end and slay the titular spire. As a roguelike game, you will go through a different path every time, with randomized encounters therein, while also gaining different upgrades along the way, thus making each run feel totally unique, but with permanent progress being gained each time towards unlocking new types of content. As a deck-building game, you will always begin with the same starting deck of basic cards, and then progressively add new cards from a wide selection to hone that deck's advanced specialization for that one run. The challenge stems from seeing a bunch of different variables thrown at you each game, figuring out how best to adapt your current strategy to fit the current situation, and trying to play your cards just right, literally, to survive long enough to stand a chance at defeating the final boss. Win or lose, you will still earn some kind of reward for your efforts, perhaps the most important being lessons learned about strategies, tactics, and mechanics that you can apply to future runs so that you can hope to do better next time.
To better understand the context for everything else I'm going to say in this review, let's start with an overview of how the game works and what you actually do in it. If you're already familiar with the basic gameplay concepts, then you can skip ahead to just get more of my thoughts on the game as a whole.