Baltimore Ravens coach John Harbaugh summed up his up-and-down defense immediately following the season like this:

"We were the best defense in the league for 12 weeks, statistically. We were toward the bottom the last four weeks," Harbaugh said in January. "That’s exactly the question we have to look at. What's it going to take to be the best defense in the league for 16 weeks-plus?"

Two months later, the Ravens aren't simply saying they want to be the best. They're taking big-money steps to get there.

Baltimore spent $58.8 million in guaranteed money to retain this year's top free agent in ESPN's rankings (nose tackle Brandon Williams), the best available safety (Tony Jefferson) and the most durable cornerback (Brandon Carr).

Only Jacksonville ($78.5 million) and New England ($63.9 million) have given more guaranteed money than the Ravens 10 days into free agency.

Defense has long been the calling card for the Ravens. Baltimore's defense has finished in the top 10 in 15 of the past 18 seasons, including last season's No. 7 finish.

The best barometer of the Ravens' success last season was their defense, which explains why 91 percent of the guaranteed money they have spent has been on that side of the ball.

The Ravens were 7-5 and in charge of their playoff fate when they were holding teams to an NFL-best 296.1 yards per game. But Baltimore allowed the fifth-most yards over the last four weeks of the regular season (400.3), and the team subsequently lost three times and missed the postseason.

Defense ultimately eliminated the Ravens from the playoffs on Dec. 25. In a 31-27 loss in Pittsburgh, Baltimore went ahead on fullback Kyle Juszczyk's touchdown with 1:18 remaining only to watch Ben Roethlisberger march 75 yards with relative ease. Steelers receiver Antonio Brown was able to stretch the ball past Eric Weddle and C.J. Mosley as well as the goal line for the winning score.

This offseason, the Ravens kept Williams, their best defensive lineman, by giving him $33.75 million guaranteed. That ranks only behind quarterback Joe Flacco on the team.

Baltimore's biggest signing of another team's free agent was Jefferson, who received $19 million in guaranteed money (third-most by a defensive back in this year's free agency). He brings more speed and physicality than corner-turned-safety Lardarius Webb.

The Ravens improved at cornerback with Carr, who is more solid than Shareece Wright and more durable than Jimmy Smith. Carr essentially signed a one-year, $6 million deal (only the $4 million signing bonus is guaranteed) with three team options after that.

Baltimore also re-signed Anthony Levine, a special teams standout who is among the team's most versatile defensive players. Levine, whose three-year deal includes $1.3 million guaranteed, can play both defensive back and linebacker.

The Ravens aren't finished on defense. They're still looking for a pass rusher to replace Elvis Dumervil, who was cut earlier this month, and inside linebacker to take over for Zach Orr, who retired at the end of the season.

"We started No. 1, and we did not finish No. 1 [in 2016]," Williams said after signing his five-year, $52.5 million contract. "So definitely, it is time to finish and get to where to where we need to go."