The Bulb is a landfill. Not in the plastic and diapers rotting in the sun sense, but in the "hey, let's take a bunch of concrete leftovers and dirt and make some more dry land!" sense. It's mainly composed of dirt, though everywhere you go, there're chunks of concrete, brick, and unidentified flotsom/jetsom around. Most of the foliage is small shrubs, only a few bushes and trees taller than 10 feet, and most of the plants are California native. The foliage is dense, and the paths are winding, making for a beautiful walk, an interesting stalk, or a horrible place to get lost.
The Bulb is used by anyone and everyone for anything and everything. I used to Society for Creative Anachronism-style fight out there with about 20 other guys (that's where the stalking comes in). People walk their dogs, people bike ride, there's a huge BMX jump on the south side of the narrow part of the bulb.
About Bum Castle, however... A friend of mine met the original bum of Bum Castle. The man was obsessed with playing cards. Bum Castle is in fact, not round, but heart shaped, and the bum made a diamond in red gravel and a club in black on the ground. My friend asked him: "So, where's the spade?" and the bum simply gestured at the rusty shovel leaning against the wall.
Minor point: The Castle is really more on the south-west side of the peninsula of the Albany Bulb.
If anyone cares to go out there, and is in the East Bay, here's the info: from the freeway (580/80), get off at Buchanan Street, and head west. You'll pass Golden Gate Fields on your left, and reach a small parking lot near a beach. this is the jumping off point. From anywhere else around there, just go to where the Albany, California Police Station is on San Pablo Avenue, turn west, go over the overpass, and go out there. The best part is, you can never be lost for long out there, you're cornered on a peninsula.