Definitly one of the most impressive monuments of Seville, which you should not miss when visiting the city, is the Alcázar Palace (los Reales Alcácares). It is a fortified palace of which the construction was ordered by Abd Al Ramán III in the year 913. Because of it's beauty it was chosen as residence by many monarchs in the centuries that followed. It now is the residence of His Royal Majesty Juán Carlos when he visits Seville.
Pedro I, "the cruel" (1350 to 1369), made significant additions to the palace which is one of the most important examples of Mudéjar architecture in Seville. The numerous rooms, patios and halls vary in architectural styles from the Islamic to Neoclassical. The beautiful gardens and fountains are especially worth visiting.
Links: en.wikipedia.org, gameofthrones.wikia.com