The Six Ancient Temples of Philaha were built and dedicated long before the time of Tactics Ogre: Let Us Cling Together. Each temple is dedicated to one of the daughters of Great Father Philaha, and thus each temple is also dedicated to one of the elements.
Distant History
Some time in the past, the temples were repurposed as fortresses to defend different parts of the Valerian Isles from foreign invaders. At this point, the temple facilities still supported worship.
Following the Great War
At the conclusion of Dorgalua's Great War, Dorgalua ordered the Apocrypha sealed. Mreuva Phoraena, familiar with the temples, sealed the Apocrypha of each element within the temple to that element. The existence of the temples was forgotten, although their use as fortresses continued.
During the Time of the Heim Conflict
Many of the fortresses have been abandoned due to the war, although they are useful enough to serve as temporary headquarters for pirates, necromancers, and the Liberation Front.
Phoraena, after joining Denam Morne's Resistance Army, revealed to Morne the secrets hidden in the old fortresses. Phoraena's daughters unsealed the temples, and the Resistance claimed the Apocrypha.
Occupants
Each temple is occupied by a Temple Guardian who serves the daughter of Philaha the temple is dedicated to. They are also filled with numerous other creatures, many of whom are associated with or comfortable in that temple's element.