Misleadingly named — it was never dedicated to Diana but was the city's imperial cult temple, dating from the late first century BC. The mistake is the fault of seventeenth-century historians; the temple itself stands more or less where the Romans built it, in a small archaeological square a couple of blocks from the Plaza de España. Its complete granite peristyle of fluted columns and Corinthian capitals is one of the most photogenic Roman remains in Spain.
The temple is partly embedded in the sixteenth-century Palacio de los Corbos, a noble residence built straight through the colonnade. The result is strange and excellent: Renaissance balconies and coats of arms hanging off Roman columns. The square is open access and free; visit in the early evening, when the lit columns glow apricot against the dark walls of the palace.