You've solved all of the Riddler's challenges, bested the Joker, and discovered who was behind the Spirit of Arkham, but have you found Batman: Arkham Asylum's ultimate secret?  As it turns out, a hidden room that's unmarked on the map and does not count towards completing the game contains some early concept art for the upcoming Batman: Arkham City.  Game Informer explains where to find this secret room and what to expect once you get inside.

Even if you've played Batman: Arkham Asylum through and through, got 100 percent clearance, and unlocked all of the achievements or trophies, you still probably haven't found Warden Sharp's secret room right off of his office. It's not located on any of the maps. You can't see the weak wall with Batman's detective vision. And you can only blow it up with several applications of explosive foam. We asked Rocksteady Studios lead narrative designer Paul Crocker how the team kept it hidden in the Internet age:

While building Batman: Arkham Asylum, we placed a number of ‘hooks’ into the game that tie into the ‘Arkham-Verse’. From the beginning we knew that Quincy was a bad guy, and planned what his next move would be. The room obviously ties into Arkham City, but to be fair, we hid it pretty well. We did assume that it would be found eventually and after following forum posts for 6 months or so, decided to announce it in the ‘Game of the Year’ podcast. The fans of the game have been amazing and again, reading the forums, we can see how much they value the efforts we have put into creating the background stories for the various characters and it was great to see all the posts and theories about the secret room once it was discovered. What I think people are really going to enjoy is not just how Arkham Asylum led to Arkham City, but also how the story in Arkham City sheds new light on the events you saw in Arkham Asylum.

I love it when games include secret hints about the next installment in the series. We don't see it very often (for obvious logistics and scheduling reasons), but on quick thought I know it happens in EarthBound. Search Fourside and you'll find a sign in front of a locked building that notes a planning meeting for EarthBound 2. Banjo-Kazooie has the infamous ice egg and other inaccessible unlockables that tease Banjo-Tooie. The Sam & Max episodic series The Devil's Playhouse has a way of showcasing future Toys of Power in small doses. The list undoubtedly goes on, although probably not for very long.