As early as the 13th century the Mongol Court (southern China) referred to the newly formed city state of Ayutthaya as “Hsien” which was pronounced something like Sayam / Siam.  Scattered throughout Thailand are ancient walled cities and huge elaborate ancient temples.  The partially restored / un-restored ancient Khmer sites are awesome .. you will often hear "WOW" (in multiple languages) at these sites!