The Perpetual City was the Unon base of operations created with the help of Time Lord technology scavenged after the Last Great Time War. The whole city was capable of travelling through the Time Vortex. It was armed with an entropy engine, which had been used to destroy Excroth.

Marleth, the leader of the surviving Excrothians, destroyed the Perpetual City with the help of the Ninth Doctor and Rose Tyler. (COMIC: Weapons of Past Destruction)

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