Air level of the map.

...How will heroes reach the heavens? Through portals, celestial ladders?...

I think it would be best for heroes to reach there naturally, i.e., after death. A hero is killed in battle – and he ends up in the heavens, the Valkyries carry him there (instead of him returning to the tavern). Living heroes have nothing to do there, except perhaps to communicate with the spirit of the deceased through some kind of magical window. And let dead heroes populate the heavens and fight with other dead heroes of the enemy.