Siege of the Saxons (1963)

"king Arthur learns one of his knights is plotting to take over and marry his daughter. Soon the soldiers of double-dealing Edmund of Cornwall slay King Arthur. However his daughter Katherine escapes with the help of outlaw Robert Marshall. Claiming she is dead Edmund makes ready to usurp the throne in league with Saxon invaders. Katherine and Robert go to find the great wizard Merlin to help them save Camelot and England. He announces that whichever Knight wishes to marry the princess must first remove King Arthur's sword from the scabbard and prove his right to the throne."

Nice example of an early 1960s Historical Adventure movie poster painting.