Wrong Interest charged

Good Morning, First up all I am thankful to you for giving opportunity to speak out here and listen to my issue. Here is my issue, I have booked apartment with Sobha, Bangalore in last June'15. I got email saying that agreement is ready and ask to pay second payment(I have paid advance while booking). But it took 1 month to reach my agreement to me and to clarify my doubts it took another month.(I.e October) . by the time I realized that I have been charged for interest for delay in payment. even sending several reminders/emails to Sobha, I haven't get any reply on wrongly interest charged. At last I got cancellation letter from them saying that they never heard out anything from me. (As I hold payment ). But when I talk to Sobha, they have threaten to me, if you won't pay they will go for cancellation and I will not get single penny as return. (Even they said legal team seating ideal if I go for court) So I decided to Pay(Principle only) , After I made payment and complaining to CRM team in no reply. I start getting response from Sobha. But still they force me to pay interest as I hold principle. I need suggestion from you on my below issue. 1. Sending agreement 1 month late. (Even after 5-6 reminders), They charged interest for that. 2. clarifying my doubts on agreement (After 5-6 reminders) , took 1 month and Also charged interest. 3. Sending around 15 emails asking for interest charged and requesting to change due date payment. No reply from them for at least (4 months) Can you just guide me , what should I do now may be legally( for not responding to my emails?) or may be wrongly interest charged?