The Gyeongbok Palace (Gyeongbokgung) was built at the end of the 14th century and served as the Royal Palace of the Joseon dynasty in the, at that time, new capital of Korea, Seoul. It is the largest of the 5 palaces in Seoul that are still standing and it is currently in a very good shape.
In the 16th century fire destroyed the palace during the Imjin War. It lasted more than 250 years before it was rebuild only to get destroyed again during the Japanese occupation that started in 1910. Since the 1990's the palace has been restored and rebuild to its current splendor.
It will take a couple of hours to see the whole palace complex. Large part of the Gyeongbok Palace compound is wheelchair friendly, however, to have a look inside most of the buildings one needs to ascend a few steps. Do not let that hold you back, the gardens are amazing and looking at Hyangwonjeong Pavilion in the middle of a pond, especially in the fall with all the colored treas, is a real treat.
The National Palace Museum of Korea and the National Folk Museum of Korea are also housed at the compound but can be visited separately.