Here's a list of secrets per level, for people hunting them down for the IL category runs!

Grizzly Hills: 1 Portrait Room, 1 Moon

Catnip Canyon: 2 Portrait Rooms, 1 Moon, 1 Secret Exit

Catnip Catacombs: 1 Portrait Room, 1 Moon

Old Garrison: 2 Portrait Rooms, 1 Moon

Wishy-Washy River: 1 Portrait Room, 1 Moon

Cloudy Meadows: 1 Portrait Room, 1 Moon

Netherworld: 1 Portrait Room, 1 Moon, 2 Secret Exits (Secret Exit 2 is 1.1+ Only), 1 Leaderboard Room (1.1+ Only)

The Tower: 1 Portrait Room, 1 Moon

The Trove: 1 Portrait Room, 1 Moon, 1 Leaderboard Room (1.1+ Only)

Dark Astoria: 1 Portrait Room, 1 Moon, 1 Secret Exit, 1 Leaderboard Room (1.1+ Only)

Steel Canyon: 1 Portrait Room, 1 Moon

Shifting Fortress: 1 Portrait Room, 1 Moon

Skies of Conflict: 1 Portrait Room, 1 Moon

Airshipvania: 2 Portrait Rooms, 1 Moon

Polyphonic Palace: 1 Portrait Room, 1 Moon, 1 Leaderboard Room (1.1+ Only)

Journey: 1 Portrait Room, 1 Moon

Folivora Cliffs: 1 Portrait Room, 1 Moon

Moisty Mountains: 1 Portrait Room, 1 Moon

Between: 1 Portrait Room, 1 Moon, 1 Leaderboard Room (1.1+ Only)

Arisen Peak: 1 Portrait Room, 1 Moon

The Purple Pinnacle: 1 Portrait Room, 1 Moon

Artemis (1.1+ Only): 1 Leaderboard Room