Was the carpet and/or doors set to be replaced anyway? And, do they have any proof of the damages they allege? If there is no supporting evidence, and the carpeting and doors were set to be replaced either way, it may be worthwhile to hire an attorney to fight. If you did cause this damage, it was more than "ordinary wear and tear", and, deductions from the deposit were within 21 days of moving out, it would be difficult to prevail - even with effective assistance of counsel.
Consult with a local landlord tenant lawyer to determine whether it makes sense to retain counsel. You should not be charged for normal wear and tear. How old was the carpeting when you vacated the premises? What shape was it in when you left? Consult with a lawyer and take all of your doucments and other information along to to your meeting with the laweyr.
If the landlord or his attorney didn't send a letter to you until April 16, he has failed to return the security deposit within 21 days as required by law. In addition to the response from the other attorneys, you will have a claim for double damages plus reasonable attorney's fees. You should hire an attorney.
Legal disclaimer: No attorney-client relationship is formed by this communication. Any recommendation/information given through Avvo should be considered only as opinion.