After the victory over the Spanish troops, who sieged the city for over a year, Leiden was granted the first University in The Netherlands in 1575. In 1587 the Univerisity asked to be allowed to establish a botanical garden, which was eventually granted by the city at 9th February 1590. The University was given about 1250 m2 for the botanical gardens.
The Hortus has grown, improved and diversified over the centuries and reaches now to the "singel", the old water defense line of the city. Nowadays species from all over the world, in glass houses or in open air are displayed. Some trees date back to the mid of the 18th century.
More info 1: http://en.wikipedia.org/wiki/Hortus_Botanicus_Leiden
More info 2: http://www.hortusleiden.nl/index.php/english/