The Pittsburgh Steelers hit free agency hard last year, bringing in a handful of veteran players to earn starting roles and improve depth.

Here is a position-by-position look at the Steelers depth chart going into this year's March 10 start of free agency.

PITTSBURGH -- The last line of defense for the Steelers is their safeties, and that group was penetrated far too often this past season with more than a dozen pass plays allowed of 40-plus yards each.

Free safety Mike Mitchell and strong safety Troy Polamalu had a lot of issues last year with pass coverage at the top. Mitchell's speed and athleticism was supposed to cover for Polamalu's free-lancing and be an upgrade to Ryan Clark. But Mitchell had a nagging groin injury from the outset last season, and that hampered his play immensely.

The Steelers must decide if the two will be paired together again this year, but many believe Polamalu will make the decision for them. He has one year remaining on what likely is his final NFL contract whether he plays or not. The Steelers will be gashed for $4.5 million against their cap, even if Polamalu is cut or retires. If he plays out the contract, he'll get $8.25 million.

Steelers 2015 Depth Chart Review Reviewing where the Steelers stand in terms of depth heading into free agency and the draft.

Polamalu's play has steadily declined, and the past two seasons he's been just average at best with rare flashes of the brilliance that punctuated his career. The Steelers probably are hopeful that he retires so they don't have to cut him. Mitchell certainly should be better if he can stay healthy. He has $19 million on the four remaining years of the free-agent he signed last year and will be worth every penny if he plays like the guy the Steelers believed they originally signed.

Mitchell had to be injured last year, because he looked nothing like the player who ran from sideline to sideline during the spring and training camp. Mitchell couldn't cover anybody deep, and he made less than a handful of big plays last season. If healthy this year, those numbers should improve immensely, and Mitchell should be a significant contributor to the Steelers secondary in 2015.

Will Allen returned to the Steelers and quickly became their third safety who played in every game with four starts and extensive play in the base defense in two others when Polamalu was out with an injury. Allen was solid, if not spectacular, with a forced fumble and quarterback pressure. But he did not have any interceptions or pass breakups. Still, he is an unrestricted free agent and could re-sign at the veteran minimum this year.

Shamarko Thomas has primarily been a special teams player, when he wasn't injured, during his opening two NFL seasons. He was pretty good in kick coverage last year, but Thomas was drafted to eventually replace Polamalu in the starting lineup. He joined his mentor to train in the offseason last year, and he'll need to make an impact on the base defense this year. Thomas had hamstring issues while Polamalu was out with an injury, so the Steelers have not been able to throw him into the fire. He has two years left on his rookie contract as a fourth-round pick in 2013 and won't get any more if he doesn't step up and make more plays this season.

Robert Golden should expect to be with the Steelers for a fourth season as a supreme special teams player, because he only played nine snaps on defense. His salary is minimal, but he was a special teams captain with 10 tackles and a forced fumble. All teams need players like Golden, and the Steelers are no exception. So, he will be back this year.

Ross Ventrone is another defensive back who made the club as a special teams performer, and he excelled at it once he got the chance. He played just nine games after starting the season on the practice squad and also spending some time injured, but he is signed for this season and should have no problems securing a roster spot. Ventrone, who played scholastically in the Pittsburgh area (Chartiers Valley High School), was a special teams gunner.

The four remaining safeties on the Steelers spring roster could get a shot to earn spots at training camp this summer. They are Jordan Dangerfield, Alden Darby, Isaiah Lewis and Ian Wild. Dangerfield spent camp last year with the Steelers, was cut and then returned midway through the season. Darby, Lewis and Wild also signed future contracts with the Steelers.

Dangerfield has the most experience with the Steelers. Darby, who played collegiately at Arizona State, spent time with the San Diego Chargers last year. Lewis had a terrific college career at Michigan State and signed last year with the Cincinnati Bengals. He's 5-foot-10 and 211 pounds. Wild is listed at 6-feet tall and 208 pounds, but he already has played professionally for the Winnipeg Blue Bombers in the Canadian Football League during the 2013-14 season. He actually was listed as a linebacker, but will be a safety in the NFL.