It was founded in the year 25 BC, with the name of Emerita Augusta (meaning the bachelors – discharged soldiers – of the army of Augustus, who founded the city; the name Mérida is an evolution of this) by order of Emperor Augustus, to protect a pass and a bridge over the Guadiana river. The city became the capital of Lusitania province, and one of the most important cities in the Roman empire. Mérida preserves more important ancient Roman monuments than any other city in Spain (including a triumphal arch of the age of Trajan).
The Archaeological Ensemble of Mérida is a UNESCO World Heritage site since 1993.
These pictures, a few more (and more yet to come) on my Merida Album on Flickr.