The oldest part of the Castle of Padua dates back to the times of Ezzelino III da Romano, tyrant of the city from 1237 to 1256. On the remains of this structure the Carraresi, lords of Padua, built their castle, which, as demonstrated by the restorations, was externally decorated in red and white squares. The defensive function of the Castle was not in the sixteenth century, when the Venetian walls were built more out of the city, and especially following the period of peace that the city enjoyed during the rule of the Venetian Republic.
In 1761 the Senate of the Republic of Venice decreed the institution of an Astronomical Observatory in order to allow teachers and students of the Patavino Studio turning to experimental practice. The task was entrusted to the abbot Toaldo (Professor of Astronomy, Geography and Meteors) and the architect Domenico Cerato, with whom he chose to turn in observatory the high tower (Torlonga) of the Old Castle, in a favorable position to comment the stars. The building, now a museum, is divided into a lower observatory (housed in the Meridian Hall) and in the upper observatory (set to 35m tall in the Hall of Figures). In the exhibition rooms of the museum there are expsed observation instruments built by English craftsmen of the eighteenth century, German and Austrian telescopes, equipment useful for the study of meteorology, and other instruments of astronomical observation built and /or improved by the mechanics of the Astronomical Observatory.