Burg Nassau stands as a distinctive ruin on the Burgberg above the Lahn; its position on a 215.4 m above sea level (NHN) rocky cone, roughly 120 metres above the river, makes it a typical example of a hilltop castle in the Rhein-Lahn-Kreis in Rheinland-Pfalz. The complex goes back to the House of Nassau, whose descendants today occupy the thrones of the Netherlands and Luxembourg.
The documentary record probably extends to the early 12th century. A 1093 charter mentioning Count Dudo of Laurenburg is regarded as doubtful; secure beginnings date to around 1100. In 1120 Ruprecht I. of Laurenburg and his brother Arnold I. acquired the hill together with a residential tower and had the castle expanded in 1124. Because the site belonged to the Bistum Worms, a protracted legal dispute arose that was formally settled in 1159 by the Archbishop of Trier: the family renounced allodial rights and were enfeoffed with the castle and lordship of Nassau; from 1160 the family used the designation "Counts of Nassau."
In the 13th century a late Romanesque palas was built (1220–1230). After the partition of the county in 1255 the castle remained common property and developed into a castle held in common by co-heirs (Ganerbenburg). In the 14th century a pentagonal bergfried of 33 metres was erected, today reconstructed; another tower mentioned in 1346 has been lost. In 1372 intra-family conflicts led to the destruction of burgmann houses. In the modern period decay set in; a 17th-century copperplate engraving still shows the palas, bergfried and gatehouse, but the remains were further damaged in World War II.
Since 1965 the site has been under state administration. From 1976 the bergfried was restored according to historical representations; later the palas was largely rebuilt, involving demolition and reconstruction that were criticised for removing elements of the Staufer period and restoring them in a neo-historical form. Today the tower houses a museum room and, on the third floor, a wedding room of the civil registry office of the city of Nassau; the palas contains a knights' hall used for events. A single Staufer-period ashlar capital with foliage decoration survives from the original triforium windows and is on display.