Thank you for using the forum. My name is ***** ***** I hope to assist you with this matter.
This is an unusual matter, but you still must work through the UD procedure (the CA Courts give you a good overview of the process that you are working in, obviously the defense that you are going to use is simply that you own the property and bring in the evidence of ownership (title to the property)).
However, given the fact that someone is attempting to attack your ownership of your home, I would recommend retaining an attorney to represent you (you can have your attorney demand costs and fees against this person (even though they really aren't allowed by statute, you can usually get these if you make a demand based on frivolous filings - a judge isn't going to be pleased someone is filing UDs on someone else's property).
If someone is making a real demand against your property, be aware the next step may be what is called a "Quiet title" suit, which is a much more complex civil procedure matter, so hiring an attorney now may be a worthwhile investment (come out of the gate aggressively).
You can find local attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).