Slay the Spire 1 has 20 ascension levels. Slay the Spire 2 has 10. In both games each level also applies every level below it.

StS2 levels

LevelNameEffect
A1Swarming ElitesThe map has 8 elite nodes instead of 5
A2Weary TravelerAncients heal 80% of your missing HP instead of all of it
A3PovertyEnemies and treasure chests drop 25% less gold
A4Tight BeltOne fewer potion slot
A5Ascender's BaneYou start with the Ascender's Bane curse
A6InflationCard removal at the Merchant starts at 100 gold and rises by 50 per use, instead of 75 and 25
A7ScarcityRare and upgraded cards appear less often
A8Tough EnemiesEnemies use their higher HP values
A9Deadly EnemiesEnemies use their higher attack values
A10Double BossAct 3 ends with two bosses

Matching levels between the games

The same penalties appear in both games at different levels.

ChangeStS1StS2
More elitesA1: elite count ×1.6A1: 5 → 8 elite nodes
Less healingA5: heal 75% of missing HP between actsA2: Ancients heal 80% of missing HP
Less goldA13: boss gold ×0.75A3: all combat and chest gold ×0.75
Fewer potion slotsA11: 3 → 2A4: one fewer slot
Starting curseA10: Ascender's BaneA5: Ascender's Bane
Pricier shopsA16: Merchant prices ×1.1 except card removalA6: card removal 100 gold, +50 per use
Fewer upgraded card rewardsA12: act 2 chance 25% → 12.5%, acts 3 and 4 50% → 25%A7: part of Scarcity
Double bossA20A10

StS1 levels with no StS2 counterpart: A6 (start at 90% of max HP), A14 (lower max HP), A15 (worse event outcomes), and the levels that make normal enemies, elites and bosses deadlier, tougher and give them harder movesets (A2 to A4, A7 to A9, A17 to A19).

Card rarity at A7 Scarcity

Card rewardRare chanceCommon chance
Normal fight3% → 1.49%60% → 61.5%
Elite10% → 5%50% → 54.9%
Merchant9% → 4.5%54% → 58.5%

Scarcity also halves how fast the rare chance grows after each card without a rare, and halves the chance of upgraded reward cards in acts 2 and 3.

How enemies scale

In StS2, every enemy has a normal value and an ascension value for its HP and for each attack. A8 switches HP to the ascension value, A9 switches attacks. Examples:

EnemyHPHP at A8
Queen400419
Aeonglass512535
Nibbit42-4644-48

In StS1, each monster has its own higher numbers at the levels that affect it: normal enemies at A2, A7 and A17, elites at A3, A8 and A18, bosses at A4, A9 and A19. For example, Jaw Worm has 40-44 HP, and 42-46 HP from A7. Each monster page lists both sets of values.