Tough defense has been a staple of the AFC North division for a very long time. From the Steel Curtain defense of the '70s Pittsburgh Steelers to the Super Bowl XXXV winning Baltimore Ravens' led by their defense in 2000. The division has been defined by players of old like Jack Lambert and Mean Joe Green to current players like Troy Polamalu, Ray Lewis and Ed Reed. These players define an entire team and the division. When teams get ready to play Baltimore or Pittsburgh, and more recently, the Bengals, they think about how to beat that defense.
Even though the AFC North teams collectively don't have the toughest strength of schedule (based on 2010 wins) they do have the top three ranking defenses in the NFL. The Cincinnati Bengals lead the NFL with the No. 1 ranked defense. The Pittsburgh Steelers are in second place and the Ravens in third.
What makes a tough defense? Stars for sure -- the Ravens wouldn't be what they are without Lewis leading them. Intimidation is a factor -- offenses plan and brace themselves for the Steelers defense who has a tradition of being physical and dominating. Depth is a factor -- the Bengals defensive line rotates so many players in and out, all of them capable of making plays, that there isn't much of a fatigue factor.
Coaching obviously makes a tough defense as well and in Cincinnati Mike Zimmer has transformed a defense that couldn't stop a single team in years past into a defense that has been able to hold even the best offenses out of the end zone.
Defenses have helped Ben Roethlisberger and Joe Flacco, who both started for their teams in their rookie defenses, win games when they were younger and the Bengals defense have helped, and will continue to help, Andy Dalton do the same.
The emergence of Zimmer and the Bengals defense over the last few years cements the idea that the AFC North is one of the toughest divisions in the NFL.