Moosburg Effretikon, Medieval castle ruin in Illnau-Effretikon, Switzerland.

Moosburg Effretikon is a medieval castle ruin located on a morainic hill and surrounded by defensive ditches. The rectangular residential tower is approximately 13 meters long and 10.5 meters wide with walls about 2.2 meters thick.

Count Hartmann IV of Kyburg built the castle around 1254 for his wife Margaretha of Savoy. After the childless couple died, it passed to the Habsburgs in 1273 and around 1300 became a fief of the Lords of Schlatt. The Zurich troops destroyed it in 1386, and in May 1444 Inner Swiss troops destroyed it again during the Old Zurich War, after which the ruin served as a quarry.

The name Moosburg refers to the marshy plain, the Moos, on whose morainic hill the castle stood. The site later served as a quarry, and a strip of the former reed bed survives east of the hill. The castle was the administrative seat of several lower courts around 1400.

The ruin stands on a moraine hill in the former Moosburg hamlet south of Effretikon. The hill was originally reached from the south over a bridge, and a broad, deep moat encircled the site, reinforced by a second ditch on the southeast. A strip of the former reed bed survives east of the hill.

The 1896 excavations by the Swiss Society for the Preservation of Historical Art Monuments yielded little; the most valuable find was an iron helmet recovered from the castle well. The larger digs of 1952 and 1953 produced pottery sherds, a crossbow bolt and fragments of stove tiles from around 1400.