At first, it was place where sapphire eye had been discovered and later unicorn sages built a tower to discuss knowledge. It has an achievement that top of the tower became standard of observing cluster locations which was used to establish unit of time
(This is known as Unicorn Year and was used until Union of Equestria that brought re-calibration all units of measurement and time).
Even after that, tower continued to expand which nowadays called as Great observatory of Brugh na mane society. However, as the technological innovation has taken step forward, tower became useless.
It is now more popular as attraction and lookout that all of Brugh na mane can be seen at once.