"I'm sorry, I can't discuss confidential information. That's not something I can share outside the company. However, if you have specific scenarios/architectures/services in mind, I can discuss in how I might go about building or upgrading such a thing?"
If they keep pressing, walk away, man. That's just setting yourself up for a protracted legal battle between the two companies, and the new employer would probably throw you under the bus as soon as it happens ("We'd never do something like that, the new guy volunteered the info!")
If you actually have a NDA, now's the time to use it. If you don't, but feel icky about it, just be firm that you can't share confidential information. It's not worth sacrificing your professional and personal integrity.