Thanks for responding. Well, the stairway was blocked, just as all other rooms. I couldn't do anything and so I've restarted.
Since I was playing again after a game where I got another ending by entering the coffin, perhaps that what made the stairs remain blocked? Maybe they aren't reset on some occasions? EDIT: I also had 0 loot.
As for endings, i've got the best one (1) first and then 2, 3, 8 and 12. 12 was cool.
EDIT: i'll retry a crownless run right now by speeding through the game.
EDIT 2: I can confirm now that making a beeline to the exit with 0 loot results in all doors either blocked or locked at the throne rooms. Collecting loot does not change this after reaching the throne rooms
EDIT 3: Just found out an important item on prison that was overlooked since rushing to the throne room doesn't involve getting there :)