Where are the secret rooms in Gungeon?

Related questions

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4. How do you unlock the gunslinger?
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Tags

1. #light
2. #reveal
3. #enemies
4. #cracks
5. #passage
6. #close
7. #through
8. #rooms
9. #starter
10. #blank
11. #explode
12. #secret
13. #pistol
14. #shoot
15. #other
16. #appear
17. #shining
18. #walls