Agree with Acraven's and Mikel's description/observations on Cáceres and Mérida, what's nice about Cáceres is that it's full of architectural eye candy, due to a fine mix of Roman, Islamic, Gothic, and Renaissance styles. On Mérida, its Roman sites are quite magnificent, but they are spread throughout the city and not centrally located as other historic sites tend to be, that will need to factored.
Know that Cáceres and Mérida are but one part of the Extremadura region, a really fascinating and historic place that is very much under the international tourist radar. In addition to those to fine towns, I'd recommend these sites to further enhance your experience, if time permits (north to south):
Monasterio de San Yuste - the idyllically monastery (still active) that Holy Roman Emperor Carlos V retired to after he was done being the most powerful man in the world ;-)
Trujillo - This town is where some of Spain’s most famous(infamous) conquistadors came from. The town has a few baroque and Renaissance palaces and an interesting Museum of Francisco Pizarro, who led the expedition that conquered the Inca Empire.
Guadalupe - Has the beautiful Real Monasterio de Guadalupe. You can tour this monastery and its collection of art, jewels, illuminated manuscripts. You can also see two cloisters, one late-14th-century Mudéjar, the other Gothic.
Badajoz - Capital of the Moorish kingdom, the Taifa of Badajoz. It has a nice Moorish historic quarter. The town is dominated by the Alcazaba, a 9th century fortified Moorish citadel.
Zafra - A white-washed walled town nicknamed "Little Sevilla", feels very Moorish. They filmed some scenes from the T.V. show Game of Thrones at the nearby Castillo de Feria, an Moorish hilltop fort.
Jerez de los Caballeros - A very picturesque small town. The old town is surrounded by a Moorish wall with six gates. After it was conquered from the Moors, the town was transferred to the Knights Templar.
Monesterio - unassuming little town that is the heart of the Jamon Iberico production in Spain and is the best at it, home of the real Museo del Jamon.
The only down side is that is can quite a bit hot in Extremadura if you go in summer, but I think that's besides the point if you will also be going to Sevilla. Hope this gives a few ideas :)