"we are going up" should be accepted - if not, report it - but IINM your second sentence would, well, require a word for "toward" - namely, felé, which isn't present in this sentence.
If you are going top down in the tree, the "felé" postposition has not been introduced yet. The question I have is how a seemingly general verb conjugation "felmenni" could mean "going upstairs" - does fel imply rising elevation?