A few years after this the Duke of Poland asked the aid of the order against the pagan inhabitants of the country that was afterward Prussia. These people were very savage and barbarous, and constantly committed horrible cruelties upon their more civilized neighbors, laying waste the country, destroying crops, carrying off cattle, burning towns, villages, and convents, and murdering the inhabitants with circumstances of extreme atrocity, often burning their captives alive as sacrifices to their gods. The grand master consulted with his chapter and with the Emperor on the proposed enterprise, and finally resolved to enter upon it, the Emperor undertaking to secure to the order any territory that they might be able to conquer and hold in Prussia. Pope Gregory IX, in 1230, gave his sanction to the expedition, and conferred on those concerned in it all the privileges accorded to crusaders. 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.