I kicked off the design by outlining the basic functions of an API gateway, mentioning key features like authentication, rate limiting, and load balancing. The interviewer nodded along but pushed me for more details on handling microservices and request routing. I got caught up in discussing service discovery instead of focusing on fault tolerance and scalability, which led to some back-and-forth that felt a bit rocky. I did manage to suggest using something like AWS API Gateway or Kong, but I could tell I didn't structure my thoughts as clearly as I could have.