A battle between crusaders and pagan Estonians fought in the course of the conquest of Livonia at the castle of Treiden (mod. Turaida, Latvia).
During the spring of 1211 the Estonians made a series of raids into the Christian-held areas of Ymera, northern Lettgallia, and the lower reaches of the river Aa (mod. Gauaja, Latvia). In the course of this fighting a force of up to 3,000-4,000 Estonians from the areas of Osel, Wiek, and Revele mounted a campaign against the castle of Treiden on the north bank of the Aa, which was held by baptized Livs. The Estonian forces assembled by land and waterways and besieged the castle until the arrival of reinforcements, consisting of members of the Order of the Sword Brethren and German crusaders from Riga.
The Estonians defended their position on a hill until they declared that they were ready to surrender and accept baptism. At night, however, they tried to escape in their ships. The crusaders then built a wooden bridge over the Aa and prevented the ships from leaving. The Estonians were forced to abandon their ships; they retreated the following night, suffering heavy losses and relinquishing a large amount of booty to the crusaders. The source for the battle is the chronicle of Henry of Livonia.