StS1's spire never changes: four acts in a fixed column. StS2 replaced the column with a draw from a pool of alternate maps — and made the acts themselves shrink the deeper you climb.
StS1: the spire is a constant
Four acts, always the same: Exordium → The City → The Beyond → The Ending. Act identity never varies, so act structure in StS1 is fully expressed by its encounter tables and map constants — there is no selection system to deconstruct.
StS2: acts are a discoverable pool
The sequel's act slots draw from Overgrowth, Underdocks, Hive, Glory. Per slot the game picks among your unlocked acts; a not-yet-discovered non-default act (Underdocks) is forced first when discovered; otherwise the pick rolls over the candidates.
Each act is parameterized data:
| Act | Rooms | Weak encounters | Rest count |
|---|---|---|---|
| Overgrowth | 15 | 3 | Gaussian(7±1) clamped 6–7 |
| Underdocks | 15 | 3 | Gaussian(7±1) clamped 6–7 |
| Hive | 14 | 2 | Gaussian(6±1) clamped 6–7 |
| Glory | 13 | 2 | uniform 5 or 6 |
Floors = rooms + 2 (boss + ancient); multiplayer subtracts one room. The acts literally shrink as you go deeper — later slots are shorter and lighter on weak fights, with rest counts drifting from Gaussian to flat-uniform by Glory.
What stays constant
Both games hard-code the same skeleton: fixed rows at map top/bottom, a boss terminus, and (StS2) an Ancient floor after the boss — the act pool varies, the act shape doesn't.
One subtlety worth knowing before any "which acts will I get" theory-crafting: a newly unlocked act is guaranteed to show up on your next run.