Is it worth visiting Zaragoza?

However, it is not mentioned in Rick's Spain guidebook or several other guidebooks I reviewed.

That's usually a good thing lol!!

But seriously, Zaragoza has more than 2,000 years of history including Roman ruins of the city walls, forum, and amphitheater, as well as the Aljafería; an 11th-century Moorish palace. It's also the home of the famous pilgrimage site of Nuestra Señora del Pilar Basilica, housing the Virgin Mary of Pilar. Now, would I totally change your schedule to fit in a visit, perhaps not, but if you are on the way between Madrid/Barcelona or Barcelona and the Basque country or vice versa, I think it makes for an excellent halfway stop for 2-3 nights, plus day trips, like Loarre Castle or Albarracín.

I think that Zaragoza is one of those undiscovered gems that Rick calls "back doors". It reminds me a lot of Barcelona of the 80s/90s, before international mass tourism really took off.

The places I had to see in the Zaragoza area for our cancelled 2020 trip included
Zaragoza - basilica, palace of the Aljaferia, plaza del Pilar, Torre del Pilar
Agreda - Convento de la concepcion, blue Nun. Mary of Jesus of Agreda. Body is in the convent.
Daroca- walled city, towers, fountain of 20 spouts
Pinares de Rodeno - cave paintings
Castillo de Monte, Aragon in Molina de Aragon

I also had the couple places Carlos mentioned, but we were going to visit them while staying in Sos Del Rey Católico and Teruel.

I agree with Jaime and Carlos, there is plenty to see in Zaragoza.

A topic that popped up every so often was about Oct. 12 the Spanish National Holiday. It is celebrated to different degrees through out Spain, but in Zaragoza, the 12th is the Pilar. In the Before Times it would have been a crazy week of fiesta. What will happen this year who knows, but I am sure there will be the big flower offerings.

I think that Zaragoza doesn't get as much good press as the weather there can be very uninviting. Hot in the summer and a cold with a biting wind in the winter.

One more site to visit in Zaragoza is the Museum of Origami, a seriously “undiscovered gem.” I forget how a Japanese craft came to be centered in a town in Spain; but the museum displays give the history, as well as showcasing many astonishing examples. I think there may be an international competition held there, periodically.
The ambience of the town is very pleasant. Many of the streets are lined with Art Nouveau/Deco buildings, and some are pedestrianized. The covered market, near the basilica, is one good example. There are plenty of small restaurants and cafes.
The Alferjeria Palace is gorgeous example of Moorish architecture, and there are other sites where Moorish remnants have been incorporated into newer buildings.
We stayed 3 nights. I recommend at least 2, but 3 would be better.

I spent a four nights in (scorching-hot) Zaragoza in late July 2016. I used it as a base for a couple of day-trips. I was surprised about how many English-speakers were walking around Zaragoza's historic center. There weren't a lot of other visitors at the Aljaferia, though.

I got a great deal on a room at the business-class Eurostars at the train station. After experiencing the weather, I understood why they were practically giving away rooms. If you must visit Zaragoza in the summer, I definitely recommend avoiding that hotel, because its restaurant closes during what is low-season for Zaragoza, and I couldn't find any others in the area. I had to subsist on a few choices from room service, including a really poor frozen pizza. The location is about a 40-minute walk from the historic center, another reason to think twice about that hotel. There was bus service, of course.

Interestingly, the Aragon region, which Zaragoza is the capital of, was named the top underrated region in Europe for 2021 by MojoTravels - https://www.youtube.com/watch?v=VOQahCXvQKE