lar en ▁region , ▁S igt una ▁found ▁itself ▁left ▁ast ern . ▁ ▁Bir ger ▁jar l ' s ▁elim ination ▁of ▁the ▁" true ▁Fol k ungs " ▁between ▁ 1 2 4 7 ▁( B attle ▁of ▁S par rs ät ra ) ▁and ▁ 1 2 5 1 ▁( B attle ▁of ▁Her rev ads bro ) ▁lead ▁to ▁increased ▁cons olid ation ▁of ▁the ▁Swedish ▁kingdom ▁and ▁the ▁introduction ▁of ▁a ▁contin ental ▁fe ud al ▁society . ▁ ▁Additionally , ▁the ▁Swedish ▁domin ion ▁expanded ▁east ▁as ▁Bir ger ▁jar l ▁and ▁T org ils ▁Kn ut sson ▁conquer ed ▁T av ast land ▁and ▁K are lia ▁( l ater ▁Finland ) ▁which ▁placed ▁present - day ▁Stockholm , ▁until ▁then ▁an ▁ins ign ific ant ▁peri pher al ▁island , ▁in ▁the ▁absolute ▁centre ▁of ▁the ▁small ▁emp ire . ▁ ▁Last ly , ▁navigation ▁on ▁Lake ▁M ä lar en ▁changed ▁dram atically ▁as ▁land ▁elev ation ▁isolated ▁it ▁from ▁the ▁Balt ic ▁Sea ▁during ▁the ▁ 1 3 th ▁century ▁and ▁made ▁the ▁stra it ▁in ▁what ▁is ▁today ▁central ▁Stockholm ▁the ▁only ▁navig able ▁passage ▁into ▁the ▁Lake ▁M ä lar en ▁region . ▁ ▁It ▁remains ▁uncertain ▁when ▁this ▁happened ▁more ▁precisely , ▁but ▁the ▁development ▁was ▁further ▁acceler ated ▁by ▁the ▁growth ▁of ▁international ▁trade ▁in ▁the ▁Balt ic . ▁ ▁The ▁streams ▁around ▁Stad sh ol men ▁can ' t ▁have ▁been ▁much ▁of ▁a ▁problem ▁for ▁Vik ing