The duel between Merlin and Arthur occurred inside Camelot after Merlin was imprisoned by Arthur following his arrival in the kingdom. The duel started when Arthur encountered Merlin, who had been released from the dungeons, across the streets and challenged him to a fight with maces. Merlin accepted, as he wanted to teach Arthur a lesson and Arthur, who had a mace for himself, gave another mace to him.
They then fought and initially, Arthur was gaining the upper hand over Merlin as he managed to corner him but Merlin used his magic to defend himself by sending objects at Arthur, who did not suspect that magic was being used. Merlin managed to trip Arthur with help of his magic and a rope, gaining the upper hand over Arthur himself, and prepared to use his mace on Arthur. However, he was interrupted by the arrival of Gaius, who had been watching the fight, which allowed Arthur to defeat him by wounding him across his shoulder with a broom.
Merlin was then caught by Arthur's guards, but he ordered them to release Merlin, saying that while Merlin was an idiot, he was brave. Before the defeated Merlin went to Gaius's chambers, Arthur said that there was something about him but could not understand what it was (The Dragon's Call).
Victory: Arthur Pendragon