The Bangkok city center that we see today is the new Bangkok. The capital city of Thailand or Siam, what the country was known back then moved to Thonburi (across the river from today's Bangkok) from Ayutthaya after the Burmese sacked the place for the umpteenth time. The Burmese continued to sack Thonburi and hence, the capital moved across the river to where it is today.
Owing to a lack of land, many residential projects are now taking place in Thonburi. The BTS or the Bangkok Sky Train has even extended across the river deep into Thonburi, sparking some very interesting projects.
Unlike the other side of the river, Thonburi is less hectic. However, nobody can escape the famous Bangkok traffic jams which you can't see from the pic below because it was taken on a quiet Sunday.