Originally built in 1448 as the king's private chapel (the equivalent of the Wat Phra Kaeo in Bangkok), Sri Sanphet is just south of the raised Grand Palace area.
The buildings were renovated in the 16th and 17th centuries. The 17m (55-ft.) bronze standing Buddha was originally cast and covered in gold in 1500 during the reign of the ninth king, Ramathipodi.
In 1767, the invading Burmese tried to melt the gold off the Buddha statue, causing a fire that destroyed both the image and the temple. The Buddha you see today is a replica.
Nearby are three Sri Lankan-style chedis, built during the 15th century to enshrine the ashes of three Ayutthaya kings.
A longer version:
Wat Phra Sri Sanphet (วัดพระศรีสรรเพชญ์)
The royal palace was located here from the establishment of
Ayutthaya in the reign of King Ramathibodi 1 (1350 A.D.) to the reign
of King Sam Phraya (1448 A.D.). Later King Borommatrailokanat
ordered a wat to be built on this site in 1448 A.D. to be used as a
monastic area.
After the reign of King Borommatrailokanat, his son King Ramathibodi II,
ordered the construction for two chedis, one of which was kept the
ashes of his father and the other those of his brother, King Borom-
marachathirat III. Another chedi was built by order of King Borom-
rachanophuttangkun. It was similarly used to house royal remains
those of King Ramathibodi II.
In 1499, a principal viharn was built. The following year, in 1500 A.D.,
King Ramathibodi II commanded the casting of a standing Buddha
image 16 meters high and covered with gold. This image, Phra
Buddha Chao Si Sanphet was the main object of veneration in the
royal viranra (hall of worship). After that time the ashes of members
of the royal family other than the kings were placed in small chedis
constructed at the site.
Wat Phra Si Sanphet was the royal chapel and as such did not
have a Sangavasa (no monks dwelt there). It was used for royal
ceremonies.
When Ayutthaya was sacked in 1767 A.D. the gold which covered
was taken by the invaders. During his reign King Rama I (1782 -
1809 A.D.) of the Ratanakosin Period ordered the transfer of the
inner core of Phra Buddha Chao Si Sanphet from Ayutthaya to Wat
Phra Chetuphon in Bangkok, and had it placed in a chedi specially
built for the purpose. Another Buddha image of importance called
Phralokanat was also brought to this wat at about the same time.