Wat Arun is something of a triumphant complex, dating back to ancient battles between the former Siam and Burma. Having fallen to the Burmese, Ayutthaya was reduced to rubble and ashes. But General Taksin and the remaining survivors vowed to march "until the sun rose again" and to build a temple there. Wat Arun, the Temple of the Dawn, was that temple. It is where the new king later built his royal palace and a private chapel.
If you climb to the top of the prang just before sunset, you are rewarded with an unforgettable view as the sun sinks over the Chao Praya River. Even if you don't plan to be doing any climbing, sunset is really the time to take in this place in all its glory.