the Byzantine advance into Armenia, the frontier around Diyār Bakr became a third sector, al-Thughūr al-Bakrīya (). In the Cilician sector, Mopsuestia (Ar. al-Maṣṣīṣa) was the first city to be re-occupied and garrisoned, already under the Umayyads, who settled 300 soldiers there in 703, a number raised under the first Abbasids to some 4,000. Adana followed in 758–760, and Tarsus in 787/8. Tarsus quickly became the largest settlement in the region and the Arabs' most important base of operations against the Byzantines, counting between 4,000 and 5,000 troops in its garrison. Other important fortresses in Cilicia, which however were little more than military outposts, were 'Ayn Zarba (Gr. Anazarbus), al-Hārūniya, founded by Harun al-Rashid, Tall Gubair and al-Kanīsat al-Sawdā. These were complemented by smaller forts dotted across the Cilician plain, holding smaller garrisons of a dozen or so men. In the more mountainous terrain of the Mesopotamian frontier zone, the main strongholds were located in the fertile parts of relatively isolated valleys, controlling the entrances of passes over the mountains: Mar'ash (Gr. Germanikeia), rebuilt already under Muawiyah I () and again under Harun al-Rashid, al-Ḥadath (Gr. Adata), likewise refortified by the first Abbasid caliphs and provided with 4,000 troops, and Malatya, which had been colonized by the Umayyads, destroyed by the Byzantines and rebuilt again and likewise garrisoned with 4,000 men in 757/8. Further fortresses of lesser importance in the Mesopotamian sector were Salaghus, Kaisum, Ḥiṣn Zibaṭra (Gr. Zapetra/Sozopetra), Sumaisaṭ (Gr. Samosata), Ḥiṣn Qalawdhiya and Ḥiṣn Ziyad. Some of the northern fortresses of the al-ʿAwāṣim province, like Dulūk or Cyrrhus, were also sometimes included in it. Further north, the relatively isolated fortress towns of Qālīqalā (Gr. Theodosiopolis, modern Erzurum) and Kamacha formed the northernmost outposts of Muslim rule. The Thughūr al-Bakrīya included, according to Qudama ibn Ja'far, Sumaisaṭ, Ḥānī, Malikyan, Gamah, Ḥaurān and al-Kilis. The caliphs repopulated the area by bringing in colonists and regular soldiers from Syria but also Persians, Slavs, Arab Christians, and people from the eastern edges of the Muslim world: settlers from Khurasan, the Turkic Sayābija tribe or Jatts (Ar. Zuṭṭ) from India. The regular troops stationed there were favoured with lower taxes (the tithe or ʿushr instead of the kharāj land tax), higher pay and small land grants (qaṭā'i). In early Abbasid times these troops numbered some 25,000, half of them drawn from Khurasan and the rest from Syria and Upper Mesopotamia. They were complemented