Living in Spain for a year, I have found that mine is an unpopular opinion for putting Avila among my top favorite cities to visit in Spain. Maybe I just happened to be in a a great mood during our November 2024 visit, but whatever the case I found Avila to be a charming place to wander around and see the beautiful architecture.
The city is jam-packed with impressive cathedrals (they boast to have one of the largest quantities in Spain) but be warned: even if they say they are free on your favorite Maps app, nearly every one of them actually charges a fee to visit inside. The repeated small costs can add up quick, and it is also difficult to fit seeing all of them into one day (without great intentionality) because they all close decently early.
As everyone on this site has mentioned, the Walls are the most important part of the site to visit (also with an early closing time) and your entry includes access to a looooong audio guide if you wish to listen to it. My wife and I also found the Church of St. Teresa and her nearby museum to be quite interesting. (We had read her book The Interior Castle before, however, so this probably increased our affection for learning more about the woman.)
Avila is one of the few locations where I feel like there was still more to see by the end of my time there and I would like to go back. Based on conversations I've had with others in Spain (and now, in looking at others' reviews on this website), I recognize that my opinion in liking it so much may be an unpopular one, so you will need to decide for yourself.