This is a Chumsungdae. It was an observatory of the Shilla kingdom. There's tons of significant numbers in its construction too - like, the bottom has 12 stones for the seasons, it has a total of 362 stones representing the days of the lunar year, etc. etc. Anyway, the bottom 12 layers are filled with sand. They would climb up a ladder to the window, and then climb a ladder inside to the top, where they would make astronomical observations... (er... that's observations of the stars... not really really big observations... I think)