I imagine it’s redundant because down is the only way you can descend.

Yes, I mean that's a more accurate translation that carries the redundancy better.

I considered it but I thought that "to descend" was too rare and formal in English to really carry over how the phrase truly feels in French. "I descend down" decidedly feels much more awkward than "je descend en bas" to me, if only because I've heard the latter a lot and the former probably never.

"I climb up" might give a somewhat better impression of how "je monte en haut" sounds in French, although I suppose that you can very much "climb down" in English so it's not a perfect match.

It's idiomatic to say "I rise up" or "I raise X up" in English, which are analogous though not translations. English just loves prepositional verbs.

Ah, I see, and both "climb up" and "climb down" are used, so it's not equivalent, you're right.

unless you're trapped in some escherian[1] world?

[1]:https://en.wikipedia.org/wiki/M._C._Escher

Yes but “descendre” and “monter” can mean other things in other contexts. So I feel the redundancy is from poor translation.

Exactly! Translation is tricky business.

Sure, although to descend feels a lot more formal than descendre does in French.