King Rama V had the Boromphiman Throne Hall built for his son and heir in 1897 (or 1903, depending on which source you believe). It was designed and built under the supervision of a foreign architect, which is why it is so entirely western in style.
The Phra Thinang Boromphiman palace
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It was the first royal building in Thailand to have a projecting portico where carriages could be pulled up directly to the door. Inside, a grand staircase leads up from the entrance to the reception hall which features high a vaulted ceiling capped with a skylight. Much of the interior decoration dates from the time of Rama VI, who only occupied the palace after ascending to the throne.
The palace has been used only briefly by succeeding kings, and now is used on rare occasions as a guest house for visiting dignitaries.