The monuments at Bost include remains from periods of ancient Iranian, Greco-Roman, and ancient Indian domination, as well as the ruins of an imposing fortress, a soaring arch with baked-brick decoration in geometric patterns, and mud-brick walls of private houses from the Islamic period. Although classicists and orientalists have collected and interpreted the notes of early geographers and historians and European travelers have been visiting the ruins of Islamic Bost, which lies on the traditional caravan track between Iran and India, since the early 19th century, archeological documentation of pre-Islamic occupation has been discovered and recorded only during the last two decades by Norman Hammond (1970), Manfred Klinkott (1974), Warwick Ball (1982), and William Trousdale (1984). In contrast to these documents, which lie hidden under the shifting sand dunes of the Sīstān desert, the monuments of the Islamic Middle Ages still dominate the desert and riverbanks. The wealth of the medieval city was attested by Arab and Persian writers. After the Mongol invasions, the wars between India and Persia in the 10th-12th/16th-18th centuries, and the final destruction of Bost by Nāder Shah in 1150/1738 the cultivated zone reverted to steppe. In 1324 Š./1945 the Helmand-Arḡandāb Valley Authority (HAVA) began a project to irrigate and reclaim the region along the banks of the two rivers for cultivation. Both ecological and archeological activities came to an end, however, with the occupation of Afghanistan by Soviet troops in 1358 Š./1979.

In series of mounds to the north and south of Bost pottery, terracotta figurines, inscribed seals, and coins datable between 500 b.c. and a.d. 500 (Hammond, 1970; Ball, 1982), that is, from the successive Achaemenid, Parthian, Kushan, Sasanian, and Chionite-Hephthalite periods, have been found. They confirm reports in Greco-Roman sources that a fortified settlement was located in the triangle formed by the confluence of the Helmand and Arḡandāb rivers. It was known to classical geographers as Bestia Deselutia, Bestigia Deselenga, Bispolis, and Biyt; Islamic geographers and Western travelers have called it variously Bist, Bost, Bust, Kala-i-Bist, Qala-i-Bust, Cala Bust, and the like (Fischer, 1967, s.v.). It served as a guard post for the caravan trade from eastern Iran up the Helmand to the point at which the road divided, one branch running to Qandahār, the other to India. Cultural influences also traveled along this route from India to Iran and the reverse.

Pending further archeological exploration, only isolated discoveries can throw some light on the early material history of Sīstān. Klinkott (1974) compared fragments of fluted stone columns, baked bricks, and architectural terracotta from Sultan Bābā Zīārat, on the northern fringes of the Rīgestān desert, with structural and decorative forms in both secular and religious buildings of the Seleucid Near East, Parthian Nisa, and Greco-Bactrian Aï Khanum (Āy Ḵānom) Bud­dhist cave dwellings and sanctuaries at Ḵāna Gowbar illustrated by Trousdale (1984), together with a stupa at Qandahār, seem to mark the farthest westward penetration of this Indian religion (see buddhism). Gherardo Gnoli, following reports by Isidor of Charax (p. 17), indications on the Roman route map Tabula Peutin­geriana (Tomaschek, 1883, pp. 207-08), and other an­cient sources, has demonstrated the existence of Zoroastrian communities in Sīstān and particularly at Bost (pp. 45-47, 78-80); C. E. Bosworth, on the other hand, has collected references on the Nestorian (Assyrian) Christian community there (1968, pp. 9-10).

In pre-Islamic Iranian history Bost and its environs were prosperous, owing to an abundance of water from the two rivers and from wells. During the early Islamic Middle Ages the area was celebrated for its fertility, its well-irrigated orchards between the rivers, and the pontoon bridge crossing the Helmand at the point where it becomes navigable as it flows southeast. The Arab geographers of the first Islamic centuries reported both commercial and intellectual activity in the town and commented on the produce of the surrounding area, which was planted with fruit trees, vineyards, and palms. From the limited ruins so far excavated it can be surmised that soon after the Muslim conquest a strong fortress was constructed to protect the town, the irrigated farm land, and the trade route. This stronghold was the center of defense for a population under constant attack from invading armies, as well as subject to frequent rebellions among the townspeople (Tārīḵ-eSīstān, s.v.; cf. tr. Gold). It was constructed of mud and baked brick; its most notable feature was a deep well in the center of the mound, with seven galleries encircling its shaft. In general the citadel and the town itself shared the fate of the entire district: The Ghurid ruler ʿAlāʾ-al­-Dīn Ḥosayn (r. 544-56/1149-61) sacked Ḡazna in 544/1149 and shortly afterward Bost, which then became a residence for the Ghurids and later the Ḵᵛārazmšāhs. After the Mongol invasion in 618/1221 and again after Timur’s death in 807/1405 the whole of Sīstān was devastated, and, though economic life recovered, it was on a smaller scale, for the manpower necessary to maintain the extensive irrigation system was lacking. Modifications still visible in the defensive walls and the system of towers are evidence that the citadel was used by the Mughals in their wars against the Persians in the 10th-12th/16th-18th centuries. In 1150/1738 Bost suffered the same fate as Qandahār, another outpost of imperial Indian rule, when its city walls were dismantled by Nāder Shah.

The ravages of time have spared precious documents of eastern Iranian Islamic architecture, architectural decoration, and calligraphy. The baked-brick “arch of Bost” is the monumental remnant of a large mosque erected under the Ghurids or possibly the Ḵᵛārazmšāhs. The soffit of the imposing arch is faced with cut-brick decoration in a pattern of interlocking lozenges, which has been somewhat spoiled by recent restorations (Schlumberger, 1978, s.v.). Fortunately, the decoration of the facade, with horseshoe-arched niches, reflecting influences from both India and eastern Iran under the Ghaznavids and Ghurids, has been comparatively well preserved. This characteristic Iranian combination of structural monumentality and intricate ornamentation survives on a smaller scale in the patterned brickwork of the octagonal dome over the nearby mausoleum of one Ḡīāṯ-al-Dīn (or Ḥosayn Shah; Sourdel-Thomine, 1956; Hill and Grabar; Crane), where seven inscribed tomb­stones from the end of the 12th to the middle of the 13th century have been deposited in modern times (Sourdel-­Thomine, 1956). Some mud-brick houses can be recognized in the ruins north of the citadel and town; they are modest but carefully constructed, with skillfully decorated ayvān courtyard houses in the style typical of Sīstān. Howard Crane and Trousdale (1972) found decorative and inscribed bricks throughout the entire district surrounding the town. As for ceramics, J.-C. Gardin has compared plain, incised, and painted types from the nearby ruins at Laškarī Bāzār and soundings in the old town of Bost with finds from Central Asian, eastern Iranian, and Indo-Pakistani sites and has identified three major periods between about 390/1000 and 616/1220, when the Mongol invasion apparently put an end to the manufacture of ceramics at Bost.