Frankly it's a good sign you're interviewing with the CTO. Most places wouldn't waste the CTO's time unless a. you're a valued candidate and b. they might need to rely on you fairly heavily. It's been my experience that the most senior person is usually trying to ascertain your ability to "stretch", and to get a "read" on you: what would you see investing to take us to the next level, how do you handle mentoring more junior folks, can you comport yourself with business folks/ non-techs, etc...