The Bulgarian forces commanded by Ivan the Russian managed to withstand the four-month siege of Plovdiv undertaken by Andronikos III Palaiologos, claimant to the Byzantine throne, and his Bulgarian ally Voysil, despot of Kopsis. In their attempts to capture the city, the Byzantines employed German specialists to construct a siege machine, from which they fired with crossbows at the Bulgarian defenders. Despite the elaborate siege tactics, by the summer of 1323 the Bulgarians had not only retained the city, but also launched raids on nearby Byzantine-held fortresses in the Rhodope Mountains, such as Stenimachos (modern Asenovgrad) and Tsepina. Plovdiv was lost, however, to the Byzantines soon thereafter. As Ivan's forces were leaving the city to meet the Bulgarian garrison that was to replace them, the pro-Byzantine inhabitants of Plovdiv opened the city gates and let a covert Byzantine detachment in.