If you're standing facing the castle from Main Street, the Rose Garden will be to the right of the castle. Head as if you're going to Tomorrowland, and there will be a little path off to the left just before the bridge into Tomorrowland. It would be a great place to propose with the castle in the background! It's a small, out of the way location with lots of rose bushes and a couple of benches, if I remember correctly. You can usually find it empty, even on the busiest of days.