Lud’s bulwark, n. — Green’s Dictionary of Slang

Lud’s bulwark n.

[statues of the mythical King Lud and his sons used to stand on this old London gate, which the king had supposedly erected in 66BC, but it was more likely a Roman gate. Brewer, Dict. of Phrase and Fable (1894), suggests Ludgate comes from OE ludgeat, a postern, while DSUE opts for Norse ludden, thick, broad]

Ludgate prison.

c.1698		B.E. Dict. Canting Crew n.p.: Lud’s-bulwark c. Ludgate Prison.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.