Wiktionary
n. an ancient Greek siege engine consisting of a wheeled tower containing machines for throwing stones and spears
Wikipedia
Helepolis (, English: "Taker of Cities") is the Greek name for a movable siege tower. The most famous one was the siege engine invented by Polyidus of Thessaly and improved by Demetrius I of Macedon and Epimachus of Athens for the unsuccessful Siege of Rhodes (305 BC), based on an earlier design used against Salamis (306 BC). It was neither the first nor the largest helepolis. According to W. Murray, the earliest helepolis was constructed by Polyadis for Philip II of Macedon in 340 BC for the siege of Perinthus. The largest recorded helepolis was used by the same Demetrius in the siege of Salamis in Cyprus in 306 BC. The name was derived from "ελείν πόλεις", meaning taking or capturing cities. Descriptions of it were written by Diodorus Siculus, Vitruvius, Plutarch, and in the Athenaeus Mechanicus.
The rest of the description applies to the Helepolis used in Rhodes in 305-306 BC, because little is known about other ones.
Usage examples of "helepolis".
But as soon as an Helepolis had been constructed, which could engage on equal terms with the loftiest ramparts, the tremendous aspect of a moving turret, that would leave no hope of resistance or mercy, terrified the defenders of the citadel into an humble submission.