Is there a park to have a picnic?

Yes

There are spots to lay down a blanket to picnic on near the castle grounds or below by the the Lake Kenoza. Best thing to do is come early as parking spaces can fill up fast especially on a warm sunny day.

The bottom area features many more spots for something like that. However there is 1 picnic table in front of the castle and I think there is something under the gazebo as well.