Rannu is a place I had never heard of before starting to explore the history of Southern Estonia, and then noting that a castle had existed there. And, indeed, “had existed” normally means that something can be discovered or at least one’s own perspective improved so I went to take a look.
The above illustrates what remains of this place now — though the lake one sees here is related to a more recent dam, it is my understanding that some structure has been in place there for a long time, and that the original burg was also defended by a moat of similar proportions (if shallower).
These lands were enfeoffed to the von Dolen’s in 1224, with a Hans von Dolen known to be the first owner. It is probable that some of the fortifications here dated to this early time, though unlikely that any stone constructions were started until the very end of the 13th century. The von Dolen’s ruled these lands until 1466 when they sold their property to Bertram of the von Tiesenhausen’s who owned the property until it’s final destruction in the Livonian War, raided during January 1588. The 15th and 16th centuries likely saw some re-fortification of the area, and it is thought that a few (or a single) larger stone structure was built here in this time to act as a keep.
In the modern day, nothing remains insofar as I could see, and the casual visitor is also barred from exploring too far due to large signs indicating that it is on private property and no one is welcome. I doubt they put those signs up due to the large number of visitors to the old bischofsburg as the township carried no signs to note their former glories, but perhaps the owners of that plot have explored a bit deeper.