During the 90s, Magic: The Gathering set off a craze of collectable card games. These required players to build a deck of cards to compete with each other. For people willing to invest the time and money to play, these games were incredibly engaging. Maybe the most enjoyable way to play was the card draft in which players would open new packs of cards and select cards one by one hoping to create an effective deck. Later games like Dominion, Ascension and Star Realms (to name games I played) dispensed with the "collectable" aspect of the game by releasing complete sets of cards and building the drafting system into the core of the game. The result is a far more approachable system that can be introduced to new players.
Slay the Spire takes that basic deck builder game and turns it into a finely tuned solo game. In theory this could be a physical game, but there's a lot going on just under the surface that makes it best suited for a computer. For instance, the enemies follow simple rules for what they do each turn, but there's a definite progression of challenge to your deck's increasing strength. Also you start collecting relics that change the basic rules of the game. Finally, there are a wide variety of status effects, potions and powers that need to be accounted for. Doing it all on a table top would be tedious and slow, but the computer makes this stuff fast.
Fast play is important too. After a few games you get to know the rhythms of combat. When to block and when to attack are the basic choices, but later you need to use your deck efficiently. Playing cards shouldn't take very long and most enemies can be defeated in a few minutes. The real challenge of the game is picking one (or zero!) cards from a draw of three at the end of every combat. In other words, building your deck.
Each game will work out a bit differently based on early draws and your choice of cards. Collecting block cards might lead you down a path of defense and waiting for chances to strike. (Ideally getting a Shield Strike card.) Collecting cards that focus on other abilities results in other types of play. The real joy of the game is discovering deck strategies that combine cards in imaginative ways. To be clear, the game design is the source of the strategies. Players merely discover them. But when you do, it feels as if you've made a scientific breakthrough.
There are now four different characters to play and each has a very different set of mechanics build into their card sets. Finding the strategies of one won't help reveal the strategies of the other characters. The enemies you face along the way seem to be the same for each character, but they get increasingly challenging and interesting. Once you play through to the end, you'll have a good idea of what you need to be prepared for in the next run. Unlike so many other games, this one doesn't seem to wear out once you've completed a run. There are always more things to unlock and discover.
I'm not sure if it's true, but it very much feels as if the game should always be beaten if the player plays well. There's a quirk where you can go back to the start of a combat by quitting the game before the final blow. I've used that feature to replay combats that I've lost decisively with the same order of cards and enemy moves. Usually the second time results in a win for me as I avoid mistakes. Still, I normally just take the lose because I can see I picked cards poorly. I never feel the game is unfair; I just feel like I made mistakes I could have avoided. A good deal of this comes from being able to see enemy actions before they are made and having a somewhat predictable enemy progression.
The game also includes a bunch of cards and relics to unlock, a daily challenge mode and plenty of statistics to chew on. It rewards replay without a whiff of grinding. And it can be played in short play sessions or while listening to something else. It's an excellent blend of simplicity and depth.