Change is constant in the NFL, as both players and coaches come and go each offseason.
Rosters are tweaked with each spring and summer as teams try to find a way to be successful year-in and year-out.
Matthew Coller of SKOR North recently wrote that sustained success is tough to accomplish in the league, which makes the recent news of Vikings linebacker Anthony Barr returning to Minnesota even sweeter for Vikings fans.
In a league in which offense has been steadily growing for years, only a few franchises have put together defensive units as consistently excellent as the Minnesota Vikings under Mike Zimmer.
The Seattle Seahawks finished in the top seven in points against six years in a row from 2011-2016 and No. 1 each season between 2012-2015. And from 2007-2012, Mike Tomlin's Pittsburgh Steelers put together a run of five top-six defenses in six seasons. Aside from those two spectacular runs, Zimmer's high rankings in points allowed of 11th, fifth, sixth, first and ninth over his first five seasons in Minnesota are nearly unmatched.
If it wasn't clear before this week that the defense's unique bond was behind the success, it became especially obvious on Thursday when four-time Pro Bowl linebacker Anthony Barr explained his decision to stick with the Vikings rather than leave in free agency.
"The culture we have developed over the last five years, it's been awesome," Barr said. "The bonds I've made with my friends, my teammates and my coaches are unbreakable. It just felt right. This is where my heart was, this is where it's always been, this is where it always will be. We have a beautiful home stadium, beautiful training facility. It's hard to beat coming to work here every day with such a great group of guys. Looking forward to getting back to work and win a Super Bowl."
Barr, who will turn 27 years old on Monday, has been a key piece of the Vikings defensive success in recent seasons.
Barr has made 71 starts in his five seasons in Minnesota and has been selected to four consecutive Pro Bowls. He has 309 total tackles (according to team stats) with 31 tackles for loss, 13.5 sacks, one interception, seven forced fumbles and four fumble recoveries in his career in Purple.
And with Vikings defensive end Everson Griffen agreeing to renegotiate his contract Thursday, the Vikings will bring back almost all of their defense from a 2018 season in which Minnesota ranked fourth in yards allowed per game and ninth in points allowed per game.
In this era of the league, that's a rare feat for the Vikings to accomplish.
Vikings LB Anthony Barr and DT Shamar Stephen met with the media after signing their new contracts with the team.
Defensive players headline remaining free agents
Most of the free-agency frenzy has passed in the NFL as dozens of players have inked deals with new teams.
Some players, such as Barr, have also chosen to return to their old teams.
But with the second wave of free agency on approach, there is a defensive flavor to ESPN's list of the top 15 free agents still available.
ESPN writer Kevin Seifert compiled the list, and had the top nine players all on the defensive side of the ball. Seifert included three offensive players in his 15-person list.
A pair of former NFC North foes for the Vikings — defensive end Ziggy Ansah and cornerback Bryce Callahan — were included in the top three.
Ansah, who played for the Lions from 2013 to 2018, was Seifert's top-ranked player.
Age and a long injury history will limit his value. Most recently, two shoulder injuries cost him nine games in 2018. But he had 48 sacks in six seasons with the Lions, and a team with a plan to use him situationally could receive big dividends.
Callahan has played in Chicago the past four seasons.
Primarily working in the slot, Callahan started 10 games in 2018 and 29 in four seasons with the Bears. He would help a team that faces highly productive slot receivers. According to Pro Football Focus, opposing quarterbacks had a 78.9 passer rating when targeting Callahan in 2018, among the five lowest in the league.
Seifert's list can be found here.