Bhuikot Killa means a fort that is built on flat land and not on any mountain. Bhui means Land in Marathi and Kot means Killa or Fort. Most forts during the Shivaji's era were built atop mountains. However, certain forts were also built on flat land (not necessarily by Shivaji). A good example is Devagiri,This is now not Remained as a Fort its now residential place for JAHAGIRDAR Family. (Daulatabad) Fort in Aurangabad district, Maharashtra.
References
Bhuikot Killa Wikipedia(Text) CC BY-SA