Slay the Spire 1 always runs the same acts in the same order. Slay the Spire 2 has two possible first acts and one fixed act each for acts 2 and 3.
StS1 acts
| Act | Name | Bosses |
|---|---|---|
| 1 | Exordium | The Guardian, Hexaghost, Slime Boss |
| 2 | The City | The Champ, Bronze Automaton, The Collector |
| 3 | The Beyond | Awakened One, Time Eater, Donu and Deca |
| 4 | The Ending | Corrupt Heart, with Spire Shield and Spire Spear as the elite |
Act 4 opens only when you hold all three keys (Ruby, Emerald and Sapphire) after beating the act 3 boss.
StS2 acts
| Act | Options | Rooms | Weak fights at the start | Rest sites | Bosses |
|---|---|---|---|---|---|
| 1 | Overgrowth | 15 | 3 | 6 or 7 | Vantom, Ceremonial Beast, The Kin |
| 1 | Underdocks | 15 | 3 | 6 or 7 | Waterfall Giant, Soul Fysh, Lagavulin Matriarch |
| 2 | Hive | 14 | 2 | 6 or 7 | The Insatiable, Knowledge Demon, Kaiser Crab |
| 3 | Glory | 13 | 2 | 5 or 6 | Queen, Test Subject, Aeonglass |
Each act adds two floors to its room count: the Ancient floor at the start and the boss floor at the end. In multiplayer every act has one room fewer.
Overgrowth and Underdocks get 7 rest sites about 61% of the time and 6 otherwise. Hive gets 6 about 61% of the time and 7 otherwise. Glory gets 5 or 6 with equal chance.
Which act 1 you get
Underdocks is locked at first. It unlocks from score accumulated across runs. In the first single-player run after it unlocks, act 1 is always Underdocks. After that, act 1 is picked at random between Overgrowth and Underdocks.
Which boss you meet
Each act lists its bosses in a fixed order. In a standard run the act uses the first boss in that order that you have never fought. Once you have seen all three, the boss is random. StS1 uses the same rule for its first three acts.
At Ascension 10 in StS2, act 3 adds a second boss, drawn from the two act 3 bosses you are not already fighting.
Ancients per act
| Act | Ancients that can appear |
|---|---|
| Act 1 | Neow |
| Act 2 | Orobas, Pael, Tezcatara |
| Act 3 | Nonupeipe, Tanx, Vakuu |
Darv is shared: once unlocked, he can appear in act 2 or act 3. See Neow and the Ancients for what each one offers.