Just like you can make them answer questions, they can make you answer questions, too. They can send the same types of discovery that you sent them back to you to answer. That often happens when the landlord gets hit with the bill for his answers to yours, and he wants to retaliate by making you answer some as well, escalating the conflict.
The responses to their discovery have to be in proper format,under oath, and address their concerns. Their questions and requests may be defective, however, so you might want to object to their question instead of answering it. This in turn requires them to go through the motion to compel process, and meet and confer, all at greater expense to the landlord, and resulting in a postponement of the trial.
You should have the lawyer help you in answering their landlord's discovery. This $300 estimate may vary greatly, depending on what is required, and how much you help. This estimate is based upon your supplying the answers to the discovery so that we can block copy and paste those answers with minor adjustments and a little consultation into the properly formatted responses.