The story
Despite the claim by the Norman-Welsh Geoffry of Monmouth in his Historia Regum Britanniae that Ludgate was so called for having been built by the ancient British king called Lud—a manifestation of the god Nodens—the name is believed by later writers to be derived from "flood gate" or "Fleet gate", from "ludgeat", meaning "back gate" or "postern", or from the Old English term "hlid-geat", meaning "postern" or "swing gate".
Etymology from Wiktionary, CC BY-SA 4.0.