All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

Can anyone help me? How do I create a hero without a castle so that he doesn't die after a week?

Similar to the principle in Heroes of Might and Magic IV, where a hero can exist without a castle. I think you need to place a city of the hero's color in some inaccessible corner (a city with no development options, not even to deduct daily income) and set the defeat condition to be the death of that hero. Something like that.