It is hard to defend, nearly as hard as the old "backdoor at spawn"-sarranid castle.
I suggest either moving attacker spawn back and replacing the towers, so the front gate can be protected by range, or replacing the wooden gate with a real gate. Currently it is simply 1min of door hacking, CHARGE, another 1min of door hacking. Win. (we had a good ammount of players right now)
Having a real gate would give attackers 2 options: Seize the walls and break the backdoors to the keep, or try to seize the gate handle (I would place it somewhere where It cant be reached in 10sec, or protected by a door itself somewhere) and rush the main doors of the keep.