Right under the castle -- amazing! Had lockers in the rooms, which was great. Nice lounges. WiFi didn't reach top floor rooms but that was no real problem. Would recommend.
Nice hostel, in a great neighborhood, good tube and bus links to airports / central London. I did my laundry and stored bags in lockers after I had checked out, which was great. I was in one of the largest rooms with over a dozen beds, which was mostly fine except that at night it got pretty warm (outdoor heat + body heat indoors). Bathrooms were super clean!
This hostel is just across the street from a big bus / metro station, which makes getting around very easy. You can also walk to the center of town from there if you wish. I was worried about the neighborhood from other reviews, but it was a very typical busy station area -- always people around even if a little seedy at night. The left luggage facilities were awesome and beds very nice. The bathrooms tended to flood a little because of poor shower drainage but it seemed to be mopped quickly.