Hi Tom...
With your fix (myśląc logicznie), I think you might have changed the logic of my broken sentence.
I wanted to say
"You can enter anywhere you like, but I don't think you can come down upstairs. That is, thinking logically. On Escher's paintings, though, everything is possible."
You changed the last sentence to something that sounds (to me) like
"If you think logically, then everything is possible on Escher's paintings."
The contrast that I wanted to make is that, logically thinking, it's impossible to come down upstairs, unless you are in an Escher painting.
Maybe this could convey that contrast:
"То jeśli myśleć logicznie. Choć na obrazach Eschera, wszystko może być."
Of course all of this might be completely wrong.. I'm just guessing things.

How would you translate to Polish what I wrote in English?
Thank you for helping.