James Fox (King Rodor) is the uncle of the actress Emilia Fox, who played Morgause in previous episodes.
The ravine that Arthur and Merlin run through and the dead end where they are cornered have been use multiple times as filming locations for this series.
In the world of "Merlin", Odin is a monarch of the Kingdom of Cornwall which us on Albion's shore. In Norse mythology, Odin is a widely revered god associated with things like healing, death, the gallows, knowledge, battle, sorcery, poetry, frenzy, and the runic alphabet. He is the husband to the goddess Frigg and the father of Thor (another God in Norse mythology).
Although Gaius is traveling with the group, he is never seen on horseback, even from a distance.
Going back as far as the Middle Ages in England "corn" was a general term for any grain crop be it wheat, oats or barley. The common medieval metaphor of people cut down like corn gave us the image of Death, the Grim Reaper, with his scythe. (One swipe of a scythe can mow down a hundred stalks of grain.)