In May 1098 Kerboğa, the Emir of Mosul, left his city to help the Turks in Antioch besieged by the Crusaders. It was now urgent for the Crusaders to take the city so as not to be squeezed between the army of
Kerboğa and that of Yağı Siyan who was defending the city.
In the meantime Bohemund, the Prince of Taranto in Italy, had "made friends" with a Muslim behind the walls. This man named Firouz was an Armenian who had converted to Islam. However, it seems that he had not been granted the social status he had expected from his conversion, and as he realised that his wife was cheating him with one of his Muslim co-religionists, he was ready hand over the city to Bohemund.
Firouz was supposed to defend the Tower of the Two Sisters. This tower on the wall was located at the torrent valley of Phyrminus on the slopes of Mount Silpius opposite the Tower of Tancred.
The piece of wall where the Tower of the Two Sister was located.
The ruin of a tower still standing, who knows, perhaps the Tower of the Two Sisters
One night some of the knights climbed the wall at the Tower of the Two Sisters and were let in by Firouz. From there the knights ran down along the wall to the St. George's Gate and the Bridge Gate and had them opened to their comrades.
The Crusaders did not manage to take the citadel on Mount Silpius, so it stayed on Muslim hands. In Antioch itself the "Christian" army was killing the Muslims and pillaging everybody. (This was in fact the standard procedure of armies back then. The Mameluke Sultan Baibars was to behave equally uncivilised when he conquered the city in 1268.)
About one week later the army of Kerboğa arrived and started to lay siege to Antioch. The Crusaders were in a desperate situation. They were under siege and whatever there may have been left to eat, had already been eaten by the Turks they had killed when taking the city. On top of it, there was no army on the way to help them. The Byzantine army had turned around when the Emperor had heard the news of Kerboğa's arrival being sure that the Franks had been butchered. The solution to the problem was presented by a man of humble origin and a bad reputation. His name was Peter Bartholomew.
Peter Bartholomew claimed that St. Andrew had appeared to him. This is how the chronicler Raoul tells us the story:
In the city, also, the quarrel did not decrease, but rather increased, for when the besieged people were in the throes of famine, as mentioned above, there arose from the army of Raymond a versatile fabricator of lies, Peter, who preached that the salvation of the people had been revealed to him in this way: "St. Andrew, the apostle," he said, "appeared to me, when I was half asleep, and spoke this command in my ear. 'Arise and announce to the people who are laboring that consolation has come from heaven, which the Lance that opened the side of the Lord will confer when it is found. It lies hidden beneath the soil within the church of St. Peter. Break the pavement at such a place (and he pointed out the place), and by digging there you will find the iron mentioned. When the horror of battle threatens, turn that against the enemy, and you will conquer through it/ Terrified, I thought that I had been deceived by a dream; and that I would not disclose it, but would remain silent forever, unless I was warned the second and the third time. The quiet of the next night was again enfolding me when the same apostle again returned, uttering the very statement which he had made before, but like one scolding and in wrath. 'Wherefore' he said, 'didst thou shun me and remain silent? Thou alone art delaying the safety of many. The people have cried out to the Lord and have been heard; and still thy negligence leaves them as if neglected. Hasten, therefore, as quickly as possible to correct this, that thou mayest continue to live.' Frightened at these words, when I had emerged from the sleep, I was at the same time more certain and more troubled; yet, still I hesitated whether to keep the secret or disclose it. In this worry I passed a whole day, and half the night, with prayer and fasting, begging the Lord for the third visit, if the first two had really been from him. The cock had twice acclaimed the morning, when, at length, just before the third crow, sleep bound my tired limbs; then without delay he who had come a first time, who had come a second time, appeared there again, ever more terrible, ever more commanding. 'Rise up, go, lazy brute, mute dog, delayer of safety and victory, menace to your fellows, solace of your enemies. Thou hast trembled with fear where there was no fear; where it is thou hast no fear.' Threats and curses still continued, when my spirit, terrified with fear at the threats, carried me away from sleep; perspiration and trembling coursed over my body at the same time, and if fire was burning one side, the other was stiff with ice. By these steps I came to teach what I had learned; you, however, fathers and brothers, do not stop to test the truth of the matter; it remains for me to point out the place for you to dig."
When this rumor was brought to the ears of Raymond, he called a council and had Peter summoned to the church of St. Peter. When asked about the place, he pointed behind the altar, true to his story, and advised them to dig; and that his words might have weight, he likewise composed his expression. They dug, but without avail; the upturned earth could not return what had not been committed to it, and what it had not received. However, the man had secreted about him an Arabic spear point, from the chance finding of which he had contrived material for his deception. Therefore, seizing the hardened, worn, and aged point, which was in form and size unlike those which we used, he was encouraged thereby to believe that people would put faith in his new creations. Accordingly, when the time for the deception came, he took a spade, jumped into the pit and, turning to a corner, said, "Here we must dig. Here lies hidden what we seek. Here it will come forth." Then, multiplying blow on blow, often and more often, he pulled forth from the dug up ground the spear which had been fraudulently dropped by him. The darkness conspired in the deception; likewise, the throng of people with the darkness, and the narrowness of the pit with the throng. But when the sound of metal striking upon metal was heard, this same fabricator of lies held out the iron and filled the excited ears of the simple with these words: "Lo, behold! Heaven promised what the earth preserved; the apostle revealed what the prayer of the people obtained!" Scarcely had he said this when they went outside and, following the trophy with hymns and chants, showered it with gifts and wrapped it up in cloth of gold. 
It should be mentioned that two other "Holy Lances" are known: one of them was kept in Italy about a hundred years before the siege of Antioch, the other had been kept in Constantinople for about four hundred years.
With this newly acquired weapon the Crusaders made ready for facin Kerboğa outside the walls of Antioch. The showdown took place on June 28.
During the battle some of the Crusaders allegedly saw St. George on a white horse coming to their aid [v] and the Muslims betook themselves to their heels. The reason was hardly the vision of the starved Crusaders. Some of the Muslim generals evidently felt that it would not be an asset to them if Kerboğa had the day. So they simply left the battlefield. Whatever the case, the Crusaders pursued them as far as the Iron Bridge after which they let the return to the places they came from.
All this had been witnessed by the Muslim garrison in the citadel on the peak of Mount Silpius. When the commander så how the situation developed he found it practical to surrender to Bohemund and convert to Catholicism.[vi]
The Citadel is on the peak of Mount Silpius towards the centre.
The small mount to the left is Mount Staurin.
The Crusader army stayed in Antioch for another six month, but in January 1099 they set out towards Jerusalem. Bohemund, however, stayed and became the first prince in the Norman Principality of Antioch that was to last until 1268 when the city was totally destroyed by the Mameluke Sultan Baibars.
 August C. Krey, The first crusade; the accounts of eye-witnesses and participants (Princeton: Princeton University Press 1921) 237 - 239 (Raoul).