It's just a ruin, and it takes a lot of imagination to transport yourself back to the Middle Ages in Schleswig-Holstein with its (robber) knights. I personally think the restoration of the keep's foundation wall was well done. Access is via the hidden turnstile to the right of the road.
Here is a copy of the plaque on site.
LINAU - A KNIGHT'S CASTLE OF THE LATE MIDDLE AGES
Due to its size and multi-part structure, Linau Castle stands out from the overall landscape of Schleswig-Holstein's castles, which, although steeped in history, are relatively inconspicuous in terms of physical evidence, it is one of the few tower hill castles known to have been equipped with a stone tower since the advent of brick in castle construction. Along with Glambek Castle on Fehmarn, it is the only castle in Schleswig-Holstein that has survived, at least as a castle ruin, with its masonry standing.
Many stories surround Linau Castle. Historical sources do mention nothing about the construction of the first castle, but they do record its destruction in 1291, enforced by Lübeck and carried out by the owners themselves. It was rebuilt in 1308, underwent an unsuccessful siege in 1312, and was finally destroyed as part of a Lübeck peace treaty in 1349. Even though the more than 600-year-old Lübeck claim that the castle was a notorious robber baron's den is still present in many people's minds today, it was actually the ancestral seat of the Scharpenberg knightly dynasty, which was very respected and wealthy at the time. Some of its members held various high-ranking positions under various territorial lords. Heyno Scharpenberg, the most colorful representative, was even the leader of the knightly retinue and a close advisor to the Danish King Waldemar IV Atterdag.