Lumbi was a castle that was built in Aman to be the third dwelling place of Melko according to the early version of the legendarium in The Book of Lost Tales.
After Melko was overthrown by Tulkas in the battle of the Silent Pools during the First Farthing Forth, the castle of Lumbi was used to imprison him after he was bound and chained. Lumbi was guarded by Gorgumoth, the hound of Mandos.
At some unknown time after, Tevildo gnawed on Melko's bonds until eventually, many centuries later, Melko was released from his imprisonment. After which, Melko was pursued up the Pine of Tavrobel by Telimektar.