The Massassi Temples were ancient temples built on Yavin IV by the ancient Sith race. They were designed to harness Force energy and channel it into the top, giving enhanced energy and strength to whoever was standing on the summit.
The Dark Lord Exar Kun died in one of the temples, and bound his spirit to the stones of them, living as a Force ghost and stalking the halls for several thousand years, until other Force-sensitive beings intruded on the Temple.
One of the Massassi Temples was used as the Rebel Alliance's headquarters until the Galactic Empire drove them out. The Alliance, while they were there, strengthened the temple to withstand aerial bombardment.
Luke Skywalker started his Jedi Academy in the same temple that housed the Alliance, taking his students there and waking the spirit of Exar Kun, who they defeated. Later, Tavion and the Disciples of Ragnos used the Scepter of Ragnos to drain the Force energy from the Temple.