The Broncos' 2020 season is almost here, and as training camp approaches, we're taking a look at each of the Broncos' position groups. In this set of previews, we'll take a deep dive into what we know about the given position group and what we still need to learn. We'll also identify a player to watch and choose a battle that bears watching. At the end of the series, we'll provide an overarching look at the best training camp battles and which players have the most to prove.

We continue the series with a look at the defensive line.

What we know:

When the Broncos entered free agency in March, there was a chance the Broncos wouldn't return a single starter. Shelby Harris and Derek Wolfe were both unrestricted free agents, and Mike Purcell was a restricted free agent. Denver moved quickly to place a second-round tender on Purcell, and while Wolfe moved on to Baltimore, Harris returned to Denver late in free agency on a one-year deal. Before Harris signed that deal, though, the Broncos made an even bigger move.

In the early stages of free agency, the Broncos targeted Texans defensive tackle D.J. Reader to anchor their defensive line. But as a bidding war increased with the Bengals, Denver turned its attention toward another option. Shortly after Reader agreed to terms with the Bengals, the Broncos dealt a seventh-round pick to Tennessee for five-time Pro Bowler Jurrell Casey. After a dominant performance against the Ravens in the divisional round of the 2019 playoffs, it's clear the 30-year-old Casey still has plenty of good football left. He is arguably the best interior defensive lineman — and certainly the most accomplished — who will play for Denver during John Elway's tenure leading the team.

In addition to Casey, Harris and Purcell, the Broncos also return 2019 third-round pick Dre'Mont Jones, who recorded 3.5 sacks and an interception as a rookie. He should see plenty of time in sub-packages and could be a breakout player in 2020. Former second-round pick DeMarcus Walker and second-year player Jonathan Harris also return to the Broncos, as do nose tackles Kyle Peko and Joel Heath. Denver drafted Arkansas product McTelvin Agim in the third round of the 2020 NFL Draft and signed veteran Christian Covington in April to round out the group.

What we need to learn:

Although the Broncos added Casey to their roster in mid-March, we still don't quite know where the five-time Pro Bowler will line up. He has yet to take the field in Vic Fangio's defense, and the coaching staff hasn't expressed to the media where it imagines Casey playing on the defense.

After reviewing film of Casey's time in Tennessee, one possible answer is that he would play outside at three-technique on early downs and then shift inside to the one-technique on third downs. On early downs, that could mean a defensive line of Casey and Shelby Harris at end and Purcell at the nose. On third downs or clear passing downs, Casey would replace Purcell in the middle, and Jones could slide in at Casey's spot at end.

We'll also have to see how much Casey plays, as he's never been an every-snap player. During the seasons in which Casey has played all 16 games — he's missed just four games over the last six years — he's averaged about 80 percent of the snaps. Should we expect that same percentage, or will his playing time go up in Vic Fangio's system? Keep in mind: Von Miller's playing time increased in Year 1 of Fangio's tenure.

Player to watch:

Dre'Mont Jones

We'll certainly be watching Casey closely to see how he will complement Miller and Bradley Chubb and to see if he is still playing at a Pro Bowl level. Jones, though, is the bigger unknown and could turn heads. The 6-foot-3, 281-pound player started just one game last season, but his defensive snaps increased drastically over the final weeks of the season; he played more than half the snaps in Denver's final two games. In the Broncos' Week 16 win over the Lions, Jones tallied 2.5 sacks, two tackles for loss, five tackles and three quarterback hits to earn AFC Defensive Player of the Week honors.

When it briefly appeared that Harris would move on in free agency, Jones seemed poised to step into the starting role. That transition may be a bit delayed now, but don't be surprised if Jones pushes Harris for the job in training camp or if he assumes it at some point during the year. The Ohio State product showed flashes of potential in 2019, and it wouldn't be a surprise to see him take a big jump in 2020.

Battle to watch:

Casey, Shelbt Harris, Purcell, Jones and Agim seem to be near locks to make the roster, which leaves Walker, Covington, Jonathan Harris, Kyle Peko and Joel Heath to battle for one spot if Denver decides to keep the traditional six defensive linemen.

Walker, a 2017 second-round pick, appeared in just 13 games over the first two seasons of his career and played very few defensive snaps during that limited playing time. After being frequently inactive in former head coach Vance Joseph's defense, though, Walker appeared in 10 games last season for Fangio. Walker took advantage of the opportunity, especially during the middle of the season. He posted four sacks — including two in a shutout win over the Titans — and had at least three tackles in four games last year.

If Walker continues to progress, he could be the likely candidate for the final roster spot, but Covington will try to push him. The sixth-year player spent 2019 in Dallas, where he appeared in all 16 games and started six as he recorded a sack and 28 tackles. A season earlier, he posted 3.5 sacks for Houston in 12 games. Covington played defensive tackle for the Cowboys, so he and Walker won't necessarily be competing for the exact same reps. Still, the competition will likely boil down to which of the two players brings more to the overall position group.