A wall at least 2 blocks wide, filled with snow golems, iron golems or dogs that attack as soon as someone destroys the wall.
It is better if you thicken the outer wall to 3-4. In the cavity are snow golems,iron golems or dogs.
If the wall is destroyed, the golems and/or the dogs run out and fight the monsters or players (for iron golems, the inner space must be 4 or more blocks wide).
Dogs don't just attack.
But when tamed
Very good idea, you could even expand it so that the "defense" is ready at the push of a button when a battle for a city (PvP) or a monster attack starts.
Super as a city wall and protection in one!
I also like
how would it be to create a 128*128 map, where you can start a PvP in MC servers, which is
- sends a player after death back to the team spawn, so he can continue to play and is not thrown out after death. And then just different maps, so a castle for example or an oriental city or a map like in World of Tanks? That would be really cool!