Improper construction by builder what can be done

I had purchased an apartment from a top grade builder and the possession was due in December (including a grace period). Till November, the builder did not allow a visit to the site. Recently, when we visited the apartment, we found that construction is not as per specifications in the construction agreement. E.g. non-sliding balcony doors instead of sliding etc). On raising the issue with the builder, their team has refused to make the required changes. There is also a difference in the quality of the model apartment vs actual unit (disregarding furnishing). The builder is also delaying the possession and asking for balance amount due during possession if we want to complete the interiors even though we cannot live there (no electricity, OC, statutory approvals). What should be my next step to make sure the promised specification is delivered?