The Castle is a building dating back to the 16th century with four corner towers. It was probably owned by Venetian merchants and was incorporated into the Botanical Garden since its foundation. Until the second half of the 1930s, it was the center of educational and research activities; its rooms housed, among other facilities, the laboratory, the Museum, the Herbarium, and the Library.
Today, it is the center of technical and administrative activities of the Garden, and its rooms host offices, various warehouses, and the Museum of Paleobotany and Ethnobotany.
The Library (not to be confused with the Historical Library of the Botanical Garden of Naples, which is hosted in a different building) includes texts and journals, mainly on agricultural topics.