I have been served a civil action summons on a property that had a mortgage that was assumed 10 years ago.

You should consult an attorney and give that attorney all information relative to the case. I am wondering if you are being sued solely for foreclosure (because the lender thinks you may have some claim to the property) or if you are also being sued for breaching a promissory note (which, according to what you say, should not be the case). Without seeing the Complaint and learning all the background facts, it is impossible to give you advice on the matter.

You need to find an experienced foreclosure litigation attorney immedately. The foreclosure process is a mess. The cases are filed by "foreclosure mills" who work robotically, filing whatever the mortgage company gives them. The mortgage company's processes are likewise robotic, handled by computers and by thousands of low level, robotic employees who have no understanding of your loan or issues related to it. It is like a riderless horse. Nevertheless, the judges tend to assume that "the bank" knows what it is doing, and gives them the benefit of the doubt, which is a huge error. So when things go wrong, it is often a battle to straighten it out, yet you cannot afford not to, since otherwise you could wind up with very serious problems, including deficiency liability.

I agree that, in order to provide you with an opinion as to your rights and options, we would need to see the actual paperwork. It could be that you may still have some potential interest (in the title) of the property that the plaintiff-bank is seeking to extinguish, which is the reason you're being pulled into the case as a defendant.