It's not a bad idea, but unfortunately, I don't know how to do it, and I'm not even sure if it's possible.

Maybe someone has already done this; I remember that on one of the maps ("Journey"), there was a mod that allowed you to use the "City Portal" spell only starting from level 35. However, this mod had a side effect – after placing that map in the maps folder, this effect also spread to all other maps (as a result, completing the "Ice and Flame" map became somewhat more difficult for me :smile09:).

I discussed this on this page (the last three messages), and an explanation is given on the next page:
/topic/10908/

As for completely banning the "Astral Gate" spell, it won't change much; with proper preparation, a hero, even without reaching level 25, can pass through them with a small army, and even without using high-level dark magic spells (at that time, I was sure that they were specifically banned on this map):
Here are saves from passing two outposts: