Ancient City Pavilion of the Enlightened. Ancient Siam also known as the Ancient City - Mueang Boran in Thai- is an historical park constructed under the patronage of Lek Viriyaphant spreading over 200 acres. Ancient Siam has been called the world's largest outdoor museum; the park features one hundred and sixteen structures of Thailand's monuments and attractions. The grounds of Ancient Siam correspond roughly to the shape of Thailand. Some of the structures are replicas of sites, while others are scaled down in size. The replicas were constructed with the assistance of the National Museum of Thailand for the sake of historical accuracy.