Walkthroughs for custom maps.

He finally figured it out. It's no coincidence that there's an oasis nearby.

Yes, you're on the right track. In 6.1.1, the purple castle will open, and you'll need to conquer it, then rescue the hero with the Grail on the cross before the purple castle dies without a castle.
After that, I dealt with the pink castle and am now stuck in a situation where either my strongest hero ends up on the Grail's cross, or I just wait for the orange castle to arrive =(

Added after 37 minutes
Near the oasis, there are prophecy maps and a lucky dove. With a cheat for infinite turns, you can reach these points. But I don't know how to accumulate that many turns.