The Karkus castle, like many other Estonian strongholds, probably was built in the place of an earlier hillfort, which in the first half of the 13th century came under the rule of the Teutonic Knights. From 1248, the teutonic vogts Hermann and Werner are already known. The stronghold managed by them was still wooden at that time and probably for this reason it fell to Lithuanian invasions several times, including in 1294 and 1366. The construction of the stone castle probably ended only at the end of the third quarter of the 14th century. It was an important administrative center and a significant rest point along the vital route connecting the south and north of Livonia. The high rank of the stronghold is also evidenced in the middle of the fifteenth century, by the raise of the rank of vogt residing in the castle, to the dignity of the land vogt. Although the castle never reached the formal rank of seat of the convent, in the fifteenth century, the commander of Fellin temporarily resided in it. It was probably then that subsequent upgrades were made, adding an additional wall line from the east. This proved to be useful in 1481, when Karkus was attacked by Muscovite troops. Unfortunately, in 1560 the fortification did not withstand the next Moscow invasion and the castle was occupied by the tsarist army. In the following years, it changed owners several times before it became a member of the Polish–Lithuanian Commonwealth in 1582. Polish governments did not last long, because at the beginning of the seventeenth century, the Swedes captured the castle, occupying it for another hundred years. Medieval fortifications were already heavily damaged, and what remained was finally destroyed in 1708, during the Great Northern War.
The castle was a large defensive complex consisting of an upper castle with three wings and two wards: a smaller inner bailey located west of the upper castle and a large southern bailey separated by a moat. The considerable size of the upper castle suggests that in addition to the chapel, there could be other conventual rooms, such as a refectory or dormitory. In the fifteenth century castle fortifications were extended, adding an additional wall line with semi-cylindrical towers from the east.
Until today, the castle has been preserved in the form of a highly advanced ruin with the best preserved outer bailey and a gatehouse. The remains of the upper castle are the least visible. In recent years, the castle area has been tidied up and adapted for sightseeing.
Borowski T, Miasta, zamki i klasztory, Inflanty, Warszawa 2010.