Around 1450 the Lords of Forchtenstein died off due to lack of a male heir and the castle was passed over to the House of Habsburg, which owned it for 170 years.

Nikolaus started to fortify the crumbling castle and refurbished it with the services of Vienna builder Simon Retacco from 1630 to 1634 and with Domenico Carlone from 1643.

This led to big orders for three masters of Kaisersteinbruch, the imperial quarry: Ambrosius Petruzzy, Pietro Maino Maderno, and Mathias Lorentisch.

In the second half of the 17th century his son Paul further extended and ornamented the castle with architect Domenico Carlone.

It became a repository for weapons, archives, chronometers, machines, exotic animal preparations and other "marvels".