The City of Gold and Lead, by John Christopher, was published in 1967 by Aladdin. It is the sequel to The White Mountains.
When Will and his friends arrived at the White Mountains, they thought everything would be okay. They’d found a safe haven where the mechanical monsters called Tripods could not find them. But once there, they wonder about the world around them and how everyone else is faring against the machines. In order to save everyone else, Will and his friends want to take down the Tripods once and for all. That means journeying to the capital of the Tripods: the City of Gold and Lead. Although the journey will be difficult, the real danger comes once Will is inside, where Tripods roam freely and humans are even more enslaved than they are on the outside. Without anyone to help him, Will must learn the secrets of the Tripods—and how to take them down—before they figure out that he’s a spy…and he can only pretend to be brainwashed for so long.