First they built out the basic systems and put in a bunch of cards (this is the build I'm talking about).
Then they sent the game out to their beta testers (and eventually their early access players) to learn how players played the game. Feedback was gathered from in game metrics including how often people used certain cards and which cards related to good/bad runs. Feedback was also gathered from their discord channel.
They then adjusted the game based on the data to make the game more balanced. One of the key things they were trying to do was have no over-powered strategy. Or if there was one, make it very rare to come across. This forces players to play differently every time they play the game.
Let's hear that again
Iterate, iterate, iterate! Build a working version, learn from testers, make adjustments. Rinse and repeat.
Watch the full video for tips on gathering data, balancing, and Slay the Spire feature development.