There is an archway behind the main (Hades) temple that leads to a locked door (see attachment). But I haven't been able to find the key to that door anywhere. I also didn't find any mention of that location on the web (the Fandom wiki for the game appears to be very incomplete unfortunately).

Where does that door lead? And where can I find the key to unlock it?

It connects to the upper water cistern.

If Duli is freed, he gives you the key.

If you don't get the golden bow, Duli's key is effectively the only way up there.

Hm... weird. I did free Duli during one iteration, but don't seem to have gotten the key (or at least not retained it).

Thanks for the info.