accomplishments of the Rurikid dynasty was the attack on Byzantium in 960. Pilgrims of the Rus' had been making the journey from Kiev to Constantinople for many years, and Constantine Porphyrogenitus, the Emperor of the Byzantine Empire, believed that this gave them significant information about the arduous parts of the journey and where travelers were most at risk, as would be pertinent for an invasion. This route took travelers through domain of the Pechenegs, journeying mostly by river. In June 941, the Rus' staged a naval ambush on Byzantine forces, making up for their smaller numbers with small, maneuverable boats. These boats were ill-equipped for the transportation of large quantities of treasure, suggesting that looting was not the goal. The raid was led, according to the Primary Chronicle, by a king called Igor. Three years later, the treaty of 944 stated that all ships approaching Byzantium must be preceded by a letter from the Rurikid prince stating the number of ships and assuring their peaceful intent. This not only indicates fear of another surprise attack, but an increased Kievan presence in the Black Sea. The most serious conflict between the two powers was the war of 968–971 in Bulgaria, but several Rus' raiding expeditions against the Byzantine cities of the Black Sea coast and Constantinople itself are also recorded. Although most were repulsed, they were concluded by trade treaties that were generally favourable to the Rus'. Rus'-Byzantine relations became closer following the marriage of the porphyrogenita Anna to Vladimir the Great, and the subsequent Christianization of the Rus': Byzantine priests, architects and artists were invited to work on numerous cathedrals and churches around Rus', expanding Byzantine cultural influence even further. Numerous Rus' served in the Byzantine army as mercenaries, most notably as the famous Varangian Guard. Military campaigns Caspian expeditions of the Rus' (864–1041) Rus'–Byzantine Wars (830–1043) 1018 Polish intervention Administrative divisions Principalities and volosts The East Slavic lands were originally divided into princely domains called zemlias, "lands", or volosts (from a term meaning "power" or "government"). A smaller clan-sized unit was called a verv, or pogost, headed by a kopa or viche. In Kievan Rus' the major administrative division became the principality, or duchy (kniazivstvo), each one the seat of a kniaz (prince or duke). From the 11th to 13th centuries the principalities were divided into volosts, from its centre usually called a pryhorod (a lesser gorod or grad,