A high medieval kingdom is ruled by a king. The king is fairly cunning and intelligent, has exactly two magical gifts:
- He ages very, very slowly
- He has physical prowess well, well beyond that of a normal man.
These mean that it's nearly impossible to kill him (poisons don't phase him, stabbing him is near impossible, and simply throwing soldiers at him would require him to be considerably outnumbered, so it would not be a stealthy affair), so he knows he's fairly safe on a personal level.
The king, however, has a bad habit of taking long (decade long) trips away from his kingdom. He knows during this time he leaves his kingdom ripe for the taking of particularly ambitious noblemen, advisors, and the ilk, and if any such man did take charge, the king would not be able to face down the entire kingdom's army to regain power... nor does he want to have to go through such efforts.
How can the king structure the government so he has reasonable security that when he returns to his kingdom he's still king? What variation of a monarchy would support this?