Merlin had never needed self defence in Ealdor.

All he had to do was flash his magic and any bullies would back off, but there weren't many of those in Ealdor anyway, so Merlin wasn't that used to violence inflicted in a place where he felt safe.
Indeed, he had never feared any physical violence from anyone in Camelot (apart from being executed from the reveal of his powers, which- touch wood- hadn't happened yet.); he would never believe Arthur or any of the knights would hurt him badly, or on purpose.

None of this meant that Merlin didn't put up a fight when Sir Drien started kicking him.

Quickly deciding that he would find it easier to bring Drien down than to get up, what with the much larger man at a vantage point above him, Merlin went for the legs.
He threw himself into Drien's shins with force he didn't know he possessed. Drien stumbled and very almost fell, but he was a trained warrior and a little close combat didn't throw him.

Seeing that Merlin was keeping low, he went for the boy's exposed upper back and neck, bringing a sharp knife-hand swiftly into the pressure point between the shoulder and just below the ear.

Merlin yelped and punched Drien's foot with his uninjured fist, yelling,

"You're a disgrace of a Lord!", in a mad attempt to distract him.

Drien retaliated with a shower of punches to Merlin's back, sending him wheezing.

"Good job I'm not one then, isn't it?" Drien shouted back, almost laughing. "Good thing I killed the real Drien and took his identity so that I could rescue my partner!"

The boy was about to attempt to uproot Drien's feet as the Lord brought his knee up to meet Merlin in the chin.

"You fool, boy! You think you could outsmart me? I'm going to kill you, then me and your 'high security prisoner' are going to escape!"

Merlin bit his tongue and his skull shook upon the impact of the kick. The shock wave lasted for a few seconds and meant Drien could shake his leg free of Merlin's feeble grip and swing it back.

He brought it down, in an arching axe kick, to Merlin's ribs, and hit him hard. Before Merlin could recover, he sent another stamp to Merlin's temple, aiming sharply and knocking him unconscious. Merlin fell to the dungeon floor and his head cracked against the stone. He didn't move.

Drien sneered down at him, victorious. He never lost that feeling of superiority when he had defeated someone already below him. It felt good.