Instructions
Walk back to the tree where you spoke with the owl. This area is a short walk back from the castle gate. On the right side of this valley is a long vine.
Climb the vine to get to the top of the valley. Walk along the linear path until you reach a fork. The right side is a dead end, which you can see from the fork. Head left.
Walk along the left path until you reach the castle gate. If you look down, you will be able to see the guard, but he can't see you because you're on top of the gate.
Walk onto the top of the stone archway, which is a component of the gate. There is a ladder here, which you can use to climb down. But jump off of the archway instead and roll to prevent damage. This will put you on the other side of the gate, past the guard.