This park has been developed on the remains of the city's defense. The park has been designed in late landscape style by Salomon van der Paauw and build from 1836 - 1842. Later upper class housing was build next to the park.
The park is a municipal monument.
Source 1: http://nl.wikipedia.org/wiki/Plantsoen_%28Leiden%29 (Dutch)
Source 2: http://gemeente.leiden.nl/over-de-stad/natuur-en-milieu/natuur-in-leiden/parken-in-leiden/plantsoen/ (Dutch)