Having provided therefore a large quantity of material of all kinds, he then built an engine called the
helepolis, which far surpassed in size those which had been constructed before it...
- Diodorus Siculus
Upon the death of Alexander the Great in Babylon, his empire almost immediately collapsed into a series of Successor Kingdoms which warred among themselves for several generations. It was a time of zeal, excess, and ingenuity, as each of the successors sought to find some way to overcome the others and reunify Alexander's empire. By 305 BC a series of battlelines were more or less drawn. The power of the Antigonid holdings in Asia Minor and Syria*, formally a kingdom since two years prior, had been damaged by the catastrophic Battle of Gaza in 312 BC, but their fortunes seemed again on the rise. The Antigonid ruler, Antigonus I Monophthalmos, and his son Demetrios I, had largely rebuilt their army. Their fleet remained supreme since Demetrios crushed Ptolemy I's fleet at Salamis** in 306 BC. Late in the year, the people of Rhodes offended Antigonus by rebuffing his offer of alliance, instead tying themselves to the Ptolemaic kingdom. Antigonus ordered his son to teach the Rhodians a lesson. Descending upon the island with four hundred ships, Demetrios initiated the Siege of Rhodes.
The events of the siege itself, as described by Diodorus, make for incredible reading; the sheer scale of the battle awed the Hellenistic world and continues to do so to modern readers. The Siege of Rhodes, however, is best remembered for the Helepolis which Demetrios built. The Helepolis, which means Citytaker, was a tremendous siege tower, the likes of which had never been seen before. Diodorus provided a detailed description of the monster; since it is as complete as anything I would be able to write about the engine, I will simply quote the section:
"Having provided therefore a large quantity of material of all kinds, he built an engine called the helepolis, which far surpassed in size those which had been constructed before it. Each side of the square platform he made almost fifty cubits [22.5 metres] in length, framed together from squared timber and fastened with iron; the space within he divided by bars set about a cubit [45cm] from each other so that there might be standing space for those who were to push the machine forward. The whole structure was movable, mounted on eight great solid wheels; the width of their rims was two cubits [90cm] and these were overlaid with heavy iron plates. To permit motion to the side, pivots had been constructed, by means of which the whole device was easily moved in any direction. From each corner there extended upward beams equal in length and little short of a hundred cubits [45 metres] long, inclining towards each other in such a way that, the whole structure being nine storeys high, the first storey had an area of forty-three hundred square feet and the topmost storey of nine hundred. The three exposed sides of the machine [one would face away from the city walls] he coverd externally with iron plates nailed on so that it would receive no injury from fire carriers. On each storey there were ports on the front, in size and shape fitted to the individual characteristics of the missiles that were to be shot forth. These ports had shutters, which were lifted by a mechanical device and which secured the safety of the men on the platforms who were busy serving the artillery; for the shutters were of hides stitched together and were filled with wool so that they would yield to the blows of the stones from the [enemy] ballistae. Each of the storeys had two wide stairways, one of which they used for bringing up what was needed and the other for descending, in order that all might be taken care of without confusion. Those who were to move the machine were selected from the whole army, three thousand four hundred men excelling in strength; some of them were enclosed within the machine while otherswere stationed in its rear, and they pushed it forward [probably in relays], the skilful design aiding greatly in its motion." (Diodorus, 20.91)
The helepolis was, it is obvious, quite a piece of work, and must have struck friend and foe alike with awe when it began to move forward. The monstrosity was the apogee of Demetrios' career at siegecraft, and it earned him the epithet of Poliorketes (sometimes spelled Poliorcetes), meaning Besieger. What do we see about it? Well, first, it was gargantuan, larger than anything of its type ever before built. Second, it was very well-protected, covered with iron plating on just about any surface that could be struck. This not only made the helepolis a really, really big tank; it also made it surprisingly difficult to set on fire, which was the accepted method of coping with siege engines through history. Vulnerable parts, such as the shutters for the innumerable ballistae, catapults and archers which fired from within it, were padded against the blows of enemy artillery. The system of stairs within made for effecient supply and movement, and on top of it the entire thing could actually turn, which was something of an accomplishment for most siege weapons larger than a trebuchet.
Unfortunately, it was being commanded by Demetrios I Poliorketes, a man who, in Plutarch's words, was "more effective in preparing an army than in handling it." Demetrios thought that his tremendous siege weapon would overawe the Rhodians into surrender, but he underestimated the tenacity of the defenders. When the helepolis approached Rhodes' city walls, Rhodian troops sallied forth or boarded the engine from the walls (the sources are unclear) and managed get at the wooden structure, which was put to the torch. When the dust settled, Demetrios' spectacular siege engine was a twisted wreck, collapsed under the weight of its armor after the main structure had burned away, and the whole tower was abandoned to the defenders.
The Rhodians initially opted to keep the ruins in place, both as a sign of respect to the magnificence of Demetrios' siege operations and as a sign of victory for the defense of Rhodes. In the end, however, they had an even better idea. The helepolis was dismantled, and the scrap was used to build the Colossus of Rhodes.
* - Beyond this level of vagueness, the actual locations of the Successor borders any time before the Battle of Ipsus are so fluid that you'd have to be insane to try and peg them down specifically.
** - Not the Salamis, where the Persian Empire was defeated two centuries before, but a city on the eastern coast of Cyprus with the same name.