Zimmer was the DC during both the Campo seasons and the Parcells seasons which is very interesting. The Cowboys switched from the 4-3 to the 3-4 and Zimmer was there during the entire process despite having no experience with the 3-4 defense.
Ironically, the Cowboys best defensive statistical season was 2003 when they were #1 in yardage allowed and #2 in points against. It was their transition year where they still played the 4-3 but began to incorporate concepts to the 3-4 which they transitioned to during the 2004 and 2005 seasons.
That 2004 season was the worst defensive season with Zimmer as the DC when he had to plug 4-3 style players in a 3-4 role.
With the exception of that 2004 season, the defense improved each season that Zimmer was the DC.
Zimmer left for Atlanta when Wade Phillips came in and assumed both the HC and DC role.