1 Month in Spain - Looking for two cities to stay in

Bilbao in the north, Malaga in the south.
or
Pamplona in the north and Cordoba in the south.

For the south, I recommend Seville and for the north, Girona.

What month? Please keep in mind that if your trip is during the summer months, Andalusia is very, very hot in the summer and may not be a good choice. Temps will most likely be in the 90's and around 100 degrees F (or higher). In that case, Malaga (on the Mediterranean) might be a better choice for the south. Might have cooler temps and you would appreciate the beach.

We have stopped at Spain's Mediterranean coastal cities a couple of times. We just better prefer inland Spain.

We liked Granada where we rented an apartment for a few days. We were surprised that accommodations and food were so economical--especially in comparison with other large European countries.

I agree that going to two cities would be nice.

Bilbao (in the Old Town part) in the north, and Granada in the south. Have a great month!

Seville is a must.
Other smaller cities, Toledo, just south of Madrid. Girona near Barcelona.

Time of year would be a critical factor to me.

Zamora, León, Logroño, Albacete, Teruel, Almería, and Zaragoza all come to mind. Each is a medium sized city not overrun with tourists, so you won’t hear English every place you go.

There are a plethora of potential combinations and permutations, a few personal suggestions for smaller cities:

In Northern Spain - Oviedo, Zaragoza, Girona, Tarragona

In Southern Spain - Cadiz, Cordoba, Cartagena, Caceres‎

Thank you all for your recommendations! I will check these all out. I'm not sure timing just yet so appreciate the warning about the temps in summertime.

Do you plan to depend primarily on public transportation? I like Teruel a great deal, but I think its public-transportation links are limited.

Yes I'll probably depend on public transportation but I'll check it out. Thank you.

Northern Spain I would recommend San Sebastián/Donostia. Was there for the first time in October. It was awesome. Great food and wonderful people.

Bilbao would be the second choice.

Northern Spain has some incredible prehistoric cave art. You might pick somewhere close to the caves. We liked Santander but it’s pretty big.

Santander doesn't have the same amount of atmospheric, older architecture you'll find in many Spanish cities because it suffered a massive fire in 1941. As someone who'd expect to spend a lot of time wandering around, soaking in the local atmosphere, I wouldn't choose Santander for two weeks. For that long a stay, I'd want a city that was more engaging. Santander does have some interesting side trips, but it's not unique in that. One of the best (for those without a rental car) is a day-trip to the Picos de Europa, but it ran only once a week in high summer (beginning in early July, I think) as of 2016.

Sitges for some downtime in a beach town. Just 25 minutes from Barcelona.
Zaragoza for a city that is ignored. The origami museum is awesome.