The dominant landmark over the town and the surrounding area is Oponice Castle, which has stretched up above the Tribec Mountains since the 13th century. The castle originally served to guard and to protect the Principality of Nitra against the Tartars and the Turks. Its history, just like the history of the town, is closely associated with a Hungarian noble Apponyi family who received the castle in 1392. The Apponyi family cared for and repaired the castle, gradually expanding it into the 17th century until finally abandoning it after a destructive fire. The caste was a bastion in 1708 for the rebellious army of Francis II Rakoczi, which also sealed its fate for destruction. Once the rebellion had been crushed by the Emperor's army, it, along with many other Slovak castles, was destroyed and left in ruins.
Currently the castle site is a picturesque castle ruin dating to the Middle Ages, offering visitors beautiful views of the Nitra River valley and the Povazsky Inovec Mountains in the distance. Oponice Castle is also associated with the legend of a treasure that was buried there by Prince Svatopluk. The legend says that Svatopluk took a keen interest in the site and chose the site to bury his treasure along with his famous sword. The treasure and the sword are to be uncovered when the Slovak people need it most in order to protect itself from extinction.