The Valjala Stronghold (Latin: Castrum Waldia, Estonian: Valjala Maalinn) was a major ringfort on the island of Saaremaa in Estonia. Established in the 12th century, at the time it was the most important Oeselian stronghold. Its surrender in 1227 finalized the crusader conquest of Estonia.

The Valjala Stronghold was founded in the 12th century, and by the time of the crusades had become the main fortress of the island of Saaremaa. It was a ringfort with a courtyard area of 3,600 m (39,000 sq ft), and the only stone castle on Saaremaa. The nearby Lõve River was an actively used waterway for the Oeselians.

Henry of Livonia referred to the leaders of Valjala as nobiles, a term usually reserved for the Western European nobility. He also stressed the stronghold's importance, describing it as follows:

... fort, called Waldia, in the midst of Oesel. Waldia is the strongest city among all those of the Oeselians.

In January 1227 the Livonian Brothers of the Sword organized a major invasion against Saaremaa, marching an army of 20,000 men over the frozen sea. The stronghold of Muhu was attacked first and completely destroyed. The crusader army then laid siege to Valjala, plundering the island in the process. The defenders of the Valjala Stronghold surrendered and accepted Christianity. This surrender concluded the crusaders’ conquest of Estonia. A mass baptism was organized and a stone chapel was built about 1 km (0.62 mi) northwest from the stronghold, marking the starting point of the sacred architectural history in Estonia.

Excavation results, including finds of iron nails, indicate that the stronghold remained in use after the crusade. It was probably demolished, possibly after failed rebellions of 1236–1241 or 1260–1261.

The stronghold ruins were first excavated in 1895 by Sergei Bogojavlenski and P. P. Stackelberg. Between 1962 and 1964 a team led by Aita Kustin conducted excavations, discovering multiple building foundations and stove floors. Archaeological finds from the stronghold included crusader crossbow bolts from the 1227 siege.

The stronghold is located about 700 m (2,300 ft) south from Valjala. It is slightly oval shaped, having a diameter about 120 m (390 ft) by 110 m (360 ft). The height of the walls is 3–6 m (9.8–19.7 ft) inside and 5–8 m (16–26 ft) outside. The courtyard includes a limestone-lined well.

• Varbola Stronghold
• Ancient Estonia