1. Better strategic location on the island of Rattanakosin - protected from attack by the river to the west, canals to the north, east and south
Thonburi Kingdom already constructed the fortifications on the west and the south of Choa Phraya River so there was a need to further the construction of moats on the east and the north.
2.Choa Phraya River was passing through the middle of Thonburi and it divided the capital into two parts.
3.Thonburi Kingdom was located on the river bend that easily caused the riverbank erosion.
4.The old palace of Thonburi Kingdom was not comfortable because the palace was pressing by temples (Wat Arun Ratchawararam or Temple of Dawn and Wat Tai Talad ) on two sides.
5.The East bank of Choa Phraya River was the low land and large for capital expansion.
In case of enemy invasion, it was difficult to attack from the east bank - just protected the attack from the west side.