Pajstun or Pajstun Castle is a ruins of a medieval castle located in Little Carpathians in Slovakia, near village Borinka. It lies at an altitude of 486 m. Administratively, it belongs to the Bratislava Region, Malacky District. Due to its proximity to the capital, Bratislava, it is a popular hiking destination of the city's inhabitants.
The Pajstun Castle was built in 13th century as part of a regional castle system aimed at defending the north-western border of the Kingdom of Hungary. One of the first known records mentioning the castle (or the village below) comes from 1314 in connection to its owner, Otto from Telesprun, many sources often, mistakenly, date the first mention of the castle to 1273. In 1390, Sigismund, the Holy Roman Emperor and King of Hungary at the time, gifted the castle to the Grafs of the nearby Svaty Jur and Pezinok.
Since 1592, the castle belonged to the influential Palffy family. However, its condition has been progressively worsening, and with the looming Turkish danger at the time, the castle has undergone major repairs around 1645, led by an Italian engineer Filiberto Luchese, which fundamentally transformed the original 13th century core of the castle. Nevertheless, the owners of the castle soon started preferring other locations of greater convenience, and Pajstun's significance - and condition - began to decline. This was aggravated by a large fire in the mid-18th century which destroyed a large part of the castle. With its importance diminished, the repairs were merely provisional. The final blow, however, came in 1810, when Napoleon's army destroyed the castle with an explosion. The destruction was deemed unnecessary, as the castle was already abandoned and posed no military threat. The last owner of the castle, Ludovit Karolyi abandoned his properties in 1945, the ruins of the Pajstun Castle along with other nearby mansions and possessions among them.