If you head away from the visitor centre with the loch on your right, there is shortly a left turn onto the Raider''s Road. As you turn into it there is a car park on your right. We overnighted there once.
You enter the observatory from the small road leading to Loch Doon, wild camping at the far end of the Loch near the castle is perfect & at this time of year the amount of stars you can see is mind blowing.
Nice wee campsite next to House O The Hill pub. There are some weight restrictions about 1.1miles from Bruce's Stone & I for one wouldn't risk crossing it in a heavy motorhome.