It is customary in olden days to establish Kotravai on important fort entrances or the main entrance of the city. Usually all major cities have 4 entrances. Forts may have 2 or more. Thus, the kings will establish a small shrine of kotravai or durga near the entrance.
She is considered to be the guardian angel - Kaaval Devathai. Warriors will pray her while going to battles.
Vijayalaya established such guardian deties - I think on 4 directions - of Thanjavore. Nisumbhasoodhani was the most famous one. Some believe patteswaram durgai, Nallur kali were also guardians of chola regime.
Tamil epics call it Kotravaik Kottam. The practice of establishing durga in north niche is derived from this.
In uttaramerur, a kotravaik kottam was established. She was called by a beautiful name : Vadavayir chelvi
She was guarding the northern entrance of the settlement
Why Durga in a chaturvedhi mangalam ? Probably because she was still considered as a guardian of the settlement. Endowments were offered to her.
The sculpture has survived the test of time. A pallava durga. Not a class as compared to Iravathaneeswaram Kotravai or even Saluvan Kuppam Mahisha mardhini - but she has a grace.