No Agreements, no appointment letter or relieving signed, still recieved legal notice from Employer.

Hi there, I worked part time for a company for little less than 3 months and only signed letter of intent (offer letter) that clearly mentioned that my employment is subject to cancellation if I don't submit the required docs before a specific date. No other clauses or terms/conditions were mentioned in the letter, however the employer kept on working with me without having the requested docs even after the date on offer letter and even forced me to work full time, without having the required docs before and after the date of appointment and without getting the appointment letter signed. I then resigned from the company and the company has denied issue my dues, but now forcing me to sign the appointment and relievering AFTER i am not working for them anymore. Furthermore, they have sent a legal notice via an advocate accussing me of impacting their business and the troubles that were caused to them as I didn't any documents and now asking for a sum of rupees. I have all the proofs and emails againt their baseless accusations. Please advise if I should go ahead and reply to that and press charges again them for harassing me after the job and without having any signed documents? Or shall I just ignore and wait for them to see what they do and then take charge against them in the court of law.