This is Jack the Giant Killer (1962).
A farmboy-turned-knight must protect a princess from the schemes of an evil wizard.
The main antagonist is the Black Prince, Pendragon, master of all witches, giants and hobgoblins. The image below shows him in the centre, accompanied by his human lackey, Garna, and several inhuman-looking witches.
Years prior to the events of the film, Pendragon and his witches ravaged the Kingdom of Cornwall but were banished by Herla the Wizard. Since then, Pendragon has dwelt in a castle on an uncharted island, scheming and awaiting the day when he could return to power in Cornwall.
Early in the film, he sends a giant to Cornwall to kidnap Princess Elaine, planning to hold her hostage and force King Mark to abdicate the throne. The giant captures Elaine at her father's castle and leaves with her, but on its way to the coast it passes through the farm of the protagonist, Jack, who manages to kill it and rescue Elaine.
King Mark rewards Jack by granting him a knighthood and asks him to secretly escort Elaine to a convent in Normandy, where he hopes she'll be safe from Pendragon. Jack and Elaine board a ship disguised as peasants, but Pendragon has a spy amongst King Mark's subordinates, so he knows where they are and is able to send some of his witches to kidnap Elaine from the ship.
When Elaine is brought to Pendragon's castle, he bewitches her to make her loyal to him. This also has the effect of transforming her clothing and making her look somewhat inhuman, with green skin and yellow eyes.
Jack later arrives at Pendragon's castle to rescue her again, but Pendragon sets a trap for him, casting an illusion on Elaine to make her look like her normal self. Jack realises Elaine is bewitched when he sees her evil-looking form reflected in a mirror, and frees her from Pendragon's spell by smashing the mirror while her reflection is visible in it.
Regarding your comparison to the Sinbad films, Jack was played by Kerwin Mathews, who played Sinbad in The 7th Voyage of Sinbad (1958), while Pendragon was played by Torin Thatcher, who played the antagonist, Sokurah, in the same film. The giants in Jack the Giant Killer are also very similar-looking to the cyclops in The 7th Voyage of Sinbad.
