Backtracking in Legend of Grimrock

Question

Is every secret you missed on previous floor always obtainable later in the game or is there any blocking factor or any floor not reachable?

EDIT: I want to know if every level is always accessible, but also if every secret is still doable later in the game. For instance:

after beating level 4 the four portals to the puzzles disappeared and if there is some secret still in there is not doable anymore.

Answer 1

Yes, you can always go back and look for the secrets you've missed.

About the question in the spoiler,

in level 4 there's a hidden lever that will make the four portals reappear and let you explore everything you've missed. It's placed in the corridor that opens up once you solve the scroll's puzzle.

Answer 2

I can only answer up to level 11 (there are 13), but at anytime can you go back to the previous levels. With the exception of:

Level 6 Trapped where you are trapped in that level till you complete it and reach the stairs that go down, then the upstairs become unblocked again.

*Update

After beating the game, you can go back at anytime up until (super end game spoiler)

Till you insert all 4 pieces of machinery into the cube and activating the end game boss.