Fulcher of Chartres, Peter Tudebode, and Raymond of Aguilers, and the anonymous Gesta Francorum. Nine letters survive relating to or from the crusading army; five of them were written while the siege was underway and another in September, not long after the city had been taken. While there are many sources the number of people on crusade is unclear because they fluctuated regularly and many non-combatants on pilgrimage accompanied the soldiers. Historian Jonathan Riley-Smith offers a rough guide, suggesting that perhaps 43,000 people (including soldiers, armed poor, and non-combatants) were involved in the siege of Nicaea in June 1097, while as few as 15,000 may have taken part in the siege of Jerusalem in July 1099. Lying on the slopes of the Orontes Valley, in 1097 Antioch covered more than and was encircled by walls studded by 400 towers. The river ran along the city's northern wall before entering Antioch from the northwest and exiting east through the northern half of the city. Mount Silpius, crested by a citadel, was Antioch's highest point and rose some above the valley floor. There were six gates through which the city could be entered: three along the northern wall, and one on each of the south, east, and west sides. The valley slopes made approaching from the south, east, or west difficult, so the most practical access route for a large number of people was from the north across flatter ground. The city's defences dated from the reign of the Emperor Justinian I in the 6th century. Though Antioch changed hands twice between then and the arrival of the crusaders in 1097, each time it was the result of betrayal rather than inadequacy of the defences. After the Byzantine Empire reconquered Antioch in 969 a programme of fortification building was undertaken in the surrounding area to secure the gains. As part of this, a citadel was built on Mount Silpius in Antioch. High enough to be separate from the city below, historian Hugh Kennedy opined that it "[relied] on inaccessibility as its main defence". At its fall to Seljuk dynasty in 1085, Antioch was the last Byzantine fortification in Syria. Yaghi-Siyan was made Governor of Antioch in 1087 and held the position when the crusaders arrived in 1097. Yaghi-Siyan was aware of the approaching crusader army as it marched through Anatolia in 1097; the city stood between the crusaders and Palestine. Though under