The Castle and Town of Châteaudun
How to get there (about 1H30 from Paris)
From Paris take the A10 towards Bordeaux-Chartres. Just after the toll, keep left, straight on leads to Chartres (35 km and a possible second visit on the way back for any tourist Stakhanovists). 33 km after the toll leave the auto route at Allaines and turn right onto the D927 towards Châteaudun on one of the straightest bits of road you're ever likely to come across. It takes you directly to the town through countryside that is rich agriculturally but otherwise totally featureless; the Russian steppes can't be too different. Once in the town, the château is clearly indicated.
Eating : The simplest solution is to take a picnic, but being France, any number of restaurants can be found, on the way or in Châteaudun itself for the more food orientated tourist.
The car park is as good a place as any to eat if you arrive between 12H00 and 14H00 when the site, which is managed by the CNMHS (Caisse Nationale des Monuments Historiques et des Sites), is closed.