Suggest new ideas here.

- Yes. Three concepts are introduced (two structures and... the Judgement of Fate));

- There's no need to lure them. If they want to enter, they enter; if they don't, they don't. I don't know how to prevent them from entering, but they shouldn't be allowed to;
- They don't disappear or enter the mirror. They approach it, like an obelisk. There's no need to fight your own copy. A calculation simply takes place (is this difficult to implement?) of the number of neutral creatures destroyed;
- Swamp heroes can be considered "good" in a relative sense. Neutral monsters from the swamp Castle can be considered neutral (there is neither Good nor Evil when destroying such monsters);
- There's nothing to do on small maps))). Although... you could lower the required level to 15;
- Levels are removed in reverse order (the last five levels gained);
There must be limitations!!!! The Valley should not be destroyed!!!

Added 11 minutes later
Another thought occurred to me... it's a bit crazy. I think it would be good if you could hide one of your Castles for four days and move it to another (random) location once per game.
For this, a hero with a level of 15 or higher must be in the Castle. The Castle disappears from the map for four days (along with the hero) and reappears on the 5th day in another (random) location on the map.