How to file a case against builder for not handling possession

9 years back, we gave our land in Kolkata to a Builder (through a Mediator, also a civil lawyer) who proposed to build 5 stored residential building on our land & in return give us 2 flats in that building. Since then, the construction hasn't been completed & the flats haven't been handed over. The mediator & the builder have stopped taking our calls all together. This situation isn't going ahead by verbal means anymore. Is there any way to take this situation ahead without filing a case? If there isn't, I would appreciate if you could walk me through the steps to file a Civil Case, duration the case may take to complete & costs associated with it. Also I would like to know if the case has to be filed in Kolkata since I live in Tripura. Since the Mediator is also a Civil Lawyer, I am expecting defense/foul play from the opponent's side as well. Awaiting expert advice & help. Thank you.