Lud, son of Heli, became a king in 73 BC. Lud built a city on the banks of the river Thames and named it Caerlud, the city of Lud. The name of the city was corrupted to Caerlundein and the Romans used this to create the name Londinium, hence London. Ludgate is supposed to mark the spot where Lud was buried.
The source for this tale is the writings of Geoffrey of Monmouth in the 12th C. He stated that Brutus, the grandson of Aeneas, founded a settlement there before Lud. This and other facts indicate that Lud is at best a mythical personage.