You can get a bus from Chittorgarh to Udaipur which is 115kms and from Udaipur you can get auto or taxi, the distance is 55kms from udaipur to Kumbalgarh fort.
It's not easy to reach Kumbhalgarh by bus. You should reach Udaipur first then to Nathdwara. It's easy for you to Book a cab or taxi from Nathdwara for Kumbhalgarh nd back bcoz bus service is very low in this region.