In 1047 Casimir, aided by his Kievan brother-in-law, started a war against Masovia and seized the land. It is probable that he also defeated Miecław's allies from Pomerania and attached Gdańsk to Poland. This secured his power in central Poland. Three years later, against the will of the Emperor, Casimir seized Bohemian-controlled Silesia, thus securing most of his father's domain. In 1054 in Quedlinburg, the Emperor ruled that Silesia was to remain in Poland in exchange for a yearly tribute of 117 kg. of silver and 7 kg. of gold.

At that time Casimir focused on internal matters. To strengthen his rule he re-created the bishopric in Kraków and Wrocław and erected the new Wawel Cathedral. During Casimir's rule heraldry was introduced into Poland and, unlike his predecessors, he promoted landed gentry over the drużyna as his base of power. One of his reforms was the introduction, to Poland, of a key element of feudalism: the granting of fiefdoms to his retinue of warriors, thus gradually transforming them into medieval knights.