The promo shows the return of Princess Mithian, which is sure to cause problems for the men of Camelot when Morgana uses her in her ploy… The episode’s detailed description, which can be found on the Merlin Wikia, reads:
Continuation of fanfic. gwen chose Lancelot over Arthur, but there's a new Guinevere in Camelot. Merlin and Arthur are in danger of the sorceress Morgana who is plotting against Uther.
This video is about the opening of new 'Merlin: The Dragon Tower' at Warwick Castle. The video features actors Anthony Head (Uther Pendragon), Katie McGrath (Morganna), and Angel Coulby (Guinevere).