In the following year an army invaded Prussia and erected a fortress at Thorn, on the Vistula, on the site of a grove of enormous oaks, which the inhabitants looked upon as sacred to their god Thor.  This was followed, in 1232, by the foundation of another stronghold at Culm.  A successful campaign followed, and the castle of Marienwerder, lower down the Vistula, was after some reverses and delays successfully built and fortified.  The grand master then established a firm system of government over the conquered country, and drew up laws and regulations for the administration of justice, for the coining of money, and other necessary elements of civilization.  Other fortified places were built which gradually developed into cities and towns.  But all this was not effected without many battles and much patient endurance, and frequent defeats and checks.

Nor did the knights forget the spiritual needs of their heathen subjects.  Mission clergy labored among them, and by their instruction, and still more by their holy, self-denying lives, they succeeded in winning many to forsake their idols and become Christians.

The order received an important accession to its ranks at this time (1237) by the incorporation into it of the ancient Order of Christ, in Livonia, which had considerable possessions.  This was followed shortly afterward by an agreement between the order and the King of Denmark, by which the former undertook the defence of the kingdom against its pagan neighbors.

In 1234 the order received into its ranks Conrad, Landgrave of Thuringia and Hesse, a man who had led a wicked and violent life, but, being brought to see his errors, made an edifying repentance, and became a Teutonic Knight, and afterward was elected grand master.  This Conrad was brother to Louis of Thuringia, who was the husband of St. Elizabeth of Hungary.  After the death of Elizabeth, the hospital at Marburg, where she had passed the latter years of her widowhood in the care of the sick, was made over to the Teutonic Knights, and after her canonization a church was built to receive her remains, and placed under the care of the order.

In 1240 the knights received an earnest petition from the Duke of Poland, for aid against the Turks, who were ravaging his dominions, and by the enormous multitude of their hosts were able to defeat any army he could bring into the field.  The knights accepted the invitation, and took part in a series of bloody and obstinate battles, in which they lost many of their number.  They had also a new enemy to encounter in the Duke of Pomerania, who had been their ally, but who now sided with the Prussians against them.  In the war that ensued the Duke was defeated, several of his strongholds were taken, and he was obliged to sue for peace.

A few years afterward, however (1243), the Duke recommenced hostilities, and with more success.  Culm was besieged by him, and the greatest miseries were endured by the inhabitants, the slaughter being so great in the numerous conflicts before the walls that at last very few men remained.  The Bishop even counselled the widows to marry their servants, that the population of the town might not become extinct.  The war was continued for several years with varying fortune, till a peace was at last concluded, principally through the mediation of the Duke of Austria.