As part of your visit to the Purana Qila area, you can see the interesting Sher Shah Gate, positioned Mathura Road, right next to the Khair-ul-Manazil mosque. Both attractions can make up a visit to the Purana Qila area and are free to visit.
Unfortunately, we found the Sher Shan Gate to be partially filled with bricks, likely to prevent its collapse as gate remains didn't look particularly stable. It was interesting to visit this attraction for a few minutes while in this area and we enjoyed thinking about what it might have been like marching into the sixth city of Delhi in the 1540s through this very gate to enter the old fort. My how things appear to be different in Delhi today.
Probably would pass on visiting this particularly attraction again when visiting Delhi but its worth a quick look-see if you plan to be in the area.