The Ostsiedlung probably started with the arrival of German monks in the entourage of Bolesław I, who spent part of his life in Thuringia, when he returned from exile in the Holy Roman Empire. These Cistercian monks from the Saxon abbey of Pforta were brought into the country by the duke to establish Lubiąż Abbey. The monks received permission to settle Germans on their possessions, which in turn were excluded from Polish law "for all time" and instead encouraged to use their own German law. This approach became exemplary for later German settlements, but the German law also replaced older Slavic and Polish laws in existing settlements. Towns were chartered with the codified German town law, most of the time either Magdeburg law or local Silesian variants like Środa Śląska/Neumarkt law ( ), which was a variant of Halle law. Existing towns received German town law often before the Mongol invasion in 1241. Examples include Wrocław, Oława, Sobótka and Środa Śląska.