It’s been a while since I played Monster Slayers, but I remember it being very similar to Dream Quest, which I played a ton.
The basics of the games are similar. You combat monsters with your deck, and build that deck outside of combat, mostly from defeating enemies or from shops. During combat there’s many ways in which your deck can change, but each combat starts with your deck as you’ve built it during the run.
One of the most significant changes to the combat mechanics, though, is that each card has an energy cost. To start with, each turn sees you at 3 energy, with a hand of 5 1-energy cards, mostly. So in contrast to monster slayers/dream quest, you cannot simply play your hand, as most often you lack the energy to. This makes card draw much less valuable, as you often do not have the energy to make use of your card advantage.
Another is that enemies are really predictable. They do not have a deck of their own, and will telegraph exactly how much damage they will do, if any. Most enemies also have a set sequence of actions, making them more like obstacles to be solved with your deck, rather than obstacles that spike in difficulty based on their draw.
The most significant change to the deck building comes from the fact that each of Spire’s three classes has a totally unique, and sizable, card pool. In contrast to monster slayers/dream quest, each of the classes has a lot more build variety. And since there’s no overlap between the classes, each has a very distinct mechanical identity.
I feel like the game is also less worried about breaking. It allows synergies that can get so busted strong that you will guarantee a win. But it’s usually balanced so that you scrape by if you play well enough.
The dev team is also amazing. Each week sees an official update, with new content, bug fixes, card tweaks. And if you opt into the beta branch, you get updates almost every work day. And the updates so far have consistently made it a better game.
The highest recommendations from me if you’re even remotely into roguelike deck builder things.