A little ways away - it'll take you 10-12 minutes to walk from Prague Castle. They only had 2 beers on tap when I visited, excluding the highly rated pils and dunkel, much to my disappointment. Service was cold. Prices are a bit high, but not too bad for the area. Worth a stop to try the highly rated pils and dunkel, but if they don't have them, this place isn't a top spot.
The staff can either be very friendly or quite rude, depending on which waiter you chance upon, like everywhere. We went here more than once and experienced both extremes of the spectrum :) but in general most waiters seem to be nice. The selection isn't overwhelming - they have 3-5 of their own, homebrewed, beers, depending on the season and as far as far as I can remember one or two Czech macros too. Their home produced beers have a couple of top 50 efforts among them, though. You can combine coming here with a visit of the historic Norbentine monastery the brewpub belongs to. The brewpub can get crowded.
2017 review: Returned with friends again, after giving numerous recommendations over the last several years. It was crowded, and we were treated like dirt / scum by the aggressive, rude staff on both floors of the restaurace part. Left without ordering anything after 30+ minutes of mindfuck. Score down from 74 to 60, and I should probably dump it down more. Really sad to see this become the worst cynical tourist trap in a way, while the beers are still so awesome (returned to get my no.6000 rating on my own, on my feet, at the brewery part, the brewer deserves that, the establishment, at least at this point, doesn't)...
2014 review: Returned with friends and encountered this really cool thing where they "resurrected" seasonals/specials for this weak around All Saints' Day. Had a great Black IPA, the only "survivor" since we were there on the last day. Enjoyed this visit too.
2009 review: Went there in early October with some friends for beers (and dessert) after some sightseeing around the castle and a lunch at the Klášterní Restaurace nearby. Was lucky enough to catch both the summer and the autumn seasonals at the same time (I guess). A nice place, warmed even on the outside. The beers were superb, the pear? pie or something similar I had (from their autumn menu) quite tasty. The location is quite nice too. All in all, I’ll be back, preferably for the other seasonals :) 5/7/9/8/7/15