Contemporaneous authors rarely dwelled on early medieval Southeastern Europe. For instance, the "Royal Frankish Annals" makes a passing reference to Abodrites living "in Dacia adjacent to the Danube near the Bulgarian border" on the occasion of their envoys' arrival in Aachen in 824. Bulgaria's territory increased under Krum ("c." 803–814), who took Adrianople and forced at least 10,000 of the town's inhabitants to settle north of the Lower Danube in 813. The ambitions of his son Omurtag (814–831) in the regions of the rivers Dnieper and Tisa are attested by two columns erected in the memory of Bulgar military leaders who drowned in these rivers during military campaigns. Emperor Arnulf sent envoys, in 894, to the Bulgarians to "ask that they should not sell salt to the Moravians", suggesting a Bulgarian control over either the Transylvanian salt mines or the roads to Moravia.