ARCHAEOLOGICAL EXPLORATIONS IN SOSTRA (Ivan Hristov – ivchristov70@abv.bg, Maria Kamisheva) Three construction periods were specified in the castellum. According to epigraphic sources, the first one was related to Cohors II Matiaca stationed in the region after AD 145. The Roman soldiers built the fortification wall, which is 1.20 – 1.30 m in width constructed of ashlars with a core structure of mortar and stones. During the reign of Maximinus Thrax and Gordian III, Cohors I Cispadana was stationed in the region. The destruction of the wall dates to the 230s and happened during the Gothic invasions. Coins minted by Nero, Nerva, Trajan, Antoninus Pius, Commodus and Septimius Severus and bronze fibulae dated to AD 175 – 225 were found in a burnt layer under the northern tower. The second period, when the fortification wall and the towers visible now were built, began at the end of the 3rd century AD, after the Gothic invasions. According to the coins, the construction works were carried out during the time of Aurelian, Probus and Diocletian. Presumably, the second period ended with the Gothic invasions during the reign of Valens. The following buildings functioned during the 4th century AD: the horreum, measuring 32 m by 11 m, built at the end of the 3rd – beginning of the 4th century AD and existing until the mid 5th century AD when it was reconstructed into dwellings; the principia; the praetorium, measuring 400 sq m; via sagularis, c. 7 m in width, discovered at the eastern and southern fortification walls. Via pretoriana is 4 m in width and it runs from the eastern gate of the castellum to the entrance of the principia. Via principalis, running between the southern and the northern gates, and via quintana were also explored. The third period began during the reign of Theodosius I and ended with the destruction of the castellum at the end of the 5th century AD. The latest coin found during the excavations was minted by Zeno. During the third period, buildings situated at the inner side of the eastern fortification wall were constructed and three of the gates of the castellum were closed.