Is thisThis is Jack the Giant Killer (1962)...?
A farmboy-turned-knight must protect a princess from the schemes of an evil wizard.
The Wikipedia page explainsmain antagonist is the plotBlack Prince, Pendragon, master of all witches, giants and hobgoblins. The image below shows him in detailthe 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 following paragraphWizard. Since then, Pendragon has dwelt in particular seems very closea castle on an uncharted island, scheming and awaiting the day when he could return to your descriptionpower in Cornwall.
Early in the film, with an evil sorcerer named Pendragon sending demonic witcheshe sends a giant to captureCornwall 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 subsequently bewitchingis 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 turnmake her into an evil witchloyal to him. There'sThis also mentionhas the effect of a mirror being smashedtransforming her clothing and making her look somewhat inhuman, with green skin and yellow eyes.
Jack later arrives at Pendragon's castle to free another female characterrescue her again, Lady Constancebut 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 one of Pendragon's spellsspell by smashing the mirror while her reflection is visible in it.
Pendragon sends demonic witches to intercept the ship. Amid the chaos, the ship's captain is murdered, and Elaine is captured. Jack wants the ship to follow the kidnappers, but the crew refuses and cast Jack and Peter, the captain's young son, overboard. At his castle, Pendragon bewitches Elaine into an evil witch loyal to him; Pendragon returns to Cornwall and confronts King Mark. Pendragon tells the king he has one week to renounce his throne so that Pendragon can rule with Elaine by his side. If the king refuses, Elaine will be killed. After Pendragon vanishes, King Mark realizes Lady Constance betrayed him. Standing before a mirror, she appears in witch form. King Mark smashes the mirror, freeing Lady Constance from Pendragon's spell.
The following video clip showsRegarding your comparison to the Sinbad films, Jack was played by Kerwin Mathews, who played Sinbad in The 7th Voyage of Sinbad (1958), while Pendragon bewitching Elainewas played by Torin Thatcher, causing her clothes to change and her eyeswho played the antagonist, Sokurah, in the same film. The giants in Jack the Giant Killer are also very similar-looking to turn yellowthe cyclops in The 7th Voyage of Sinbad.