Diagonally up allows you to jump off ladders. To enter a corridor directly next to a ladder, you should be positioned just below and push diagonally up.
I can't figure it out. It def would be more intuitive that moving the JOY left or right on ladder (or rope!) the hero would try to left the ladder in that direction.
However a really nice port, is the vases can be broken somehow?
Once I jumped on the rope on the start screen and left the screen up hoping in some easter egg :D
Just tried the Spelunky classic ( an online version of it https://yancharkin.github.io/SpelunkyClassicHDhtml5/ ) and now I see why is it that way.