Years ago you could camp right on the lake in primitive walk in spots, now that is not allowed.  There is a small campground with just a few spots down the road a quarter mile but it's rare to find an empty spot there.  The lake has grown in popularity over the years and if you want to secure a day spot on the lake's edge plan on getting there first thing in the morning.  People do also disperse camp about another mile down the road in a gravel pull out area.  Castle Lake is easy to get to via Castle Lake road which is paved the entire way.