It is said that the Iron Gate was built by Justinian, but he may just have improved it. It is situated at the gorge between Mount Cassius and Mount Staurin. Procopius, a Byzantine scholar from the sixth century, gives us the following description:
“It is proper to describe also what he did with the torrent which comes down from these mountains. Two precipitous mountains rise above the city, approaching each other quite closely. Of these they call the one Orocassias [Mount Cassius] and the other is called Staurin. Where they come to an end they are joined by a glen and ravine which lies between them, which produces a torrent, when it rains, called Onopnictes. This, coming down from a height, swept over the circuit-wall and on occasion rose to a great volume, spreading into the streets of the city and doing ruinous damage to those who lived in that district. But even for this the Emperor Justinian found the remedy, in the following way: Before that part of the circuit-wall which happens to lie nearest to the ravine out of which the torrent was borne against the fortifications, he built an immense wall or dam, which reached roughly from the hollow bed of the ravine to each of the two mountains, so that the stream should no longer be able to sweep on when it was at full flood, but should collect for a considerable distance back and form a lake there. And by constructing sluice-gates in this wall he contrived that the torrent, flowing through these, should lose its force gradually, checked by this artificial barrier, and no longer violently assault the circuit-wall with its full stream, and so overflow it and damage the city, but should gently and evenly glide on in the manner I have described and, with this means of outflow, should proceed through the channel wherever the inhabitants of former times would have wished to conduct it if it had been so manageable.” 
The Iron Wall – or what is left of it – is still to be seen up behind Mount Staurin where the spurious Church of St. Peter is located. This grotto attracts a lot more attention that the real thing some hundred metres behind it. You hardly meet any tourist if you visit the Iron Gate.
Mankind is more interested in fakes than facts.
 “Donkey-Drowner”, the nickname of the torrent Parmenius. During the winter this torrent swelled as it came rushing down between the two mountains and swept away anything in front of it.
 Procopius, Buildings – Book 2, Ch. 10:15-18. Evidently there already was some device for that purpose at the place. But most of the masonry is from the time of Justinian. – Glanville Downey: A History of Antioch in Syria, Princeton 1961, p. 551