The "Old Loo" is a small 15th-century moated castle a few hundred metres behind Paleis Het Loo, in the woods of the royal estate. William III bought it as a hunting lodge in 1684 and it was his summer residence while the grander palace was being built; today it is occasionally used by the Dutch royal family and is open to the public only on a limited number of guided tours each year.
Even without going inside, the walk to Het Oude Loo through the formal gardens and the wooded estate is worth doing as part of a Paleis Het Loo visit. The contrast between the strict French baroque of the palace and the older, rougher Dutch castle behind it tells you a lot about the changing tastes of the House of Orange.