A first mention of a castle in Simmern dates back to 1311, still under the rule of Raugrafen. In 1358, the castle, town and lordship came to the Palatine counts. 1410 received Stefan von Pfalz-Simmern-Zweibrücken at a division Simmern. The castle became the seat of the newly founded Duchy of Palatinate-Simmern. Stefan's son, Duke Frederick I built the castle into a castle. The castle was from 1410 to 1598 and from 1610 to 1673 residence of the Simmerner dukes. For the year 1595, 125 persons have been proven to be courtiers. 1655 major repairs were carried out. The bills issued for that time have been preserved. Among other things, 280 windows and numerous furniture had to be rebuilt.
During the Palatinate War of Succession, the city of Simmern was almost completely destroyed. Already in May 1689, the city walls were laid down and destroyed the large watchtower and burned down all the houses. The destruction of the city ended on 17 September 1689 with the demolition of the castle.
The present New Castle was built from 1708 to 1713 as the seat of a Palatinate Oberamtmannes. Napoléon donated it to the city in 1802. From then on it was used as court, school, garrison, arrest house and official residence.