Deep in the Ilex Forest sit four long walls of Forretress arranged as an open box, stacked upon each other like a reef of Corsola. Although they are calm enough that tourists can walk straight up to these walls, they remain fierce guardians who relentlessly protect the integrity of their wall and whatever lies within. Those who tried Electrode found another wall behind the first one, those who tried fire found each Forretress equipped with Occa berries; all of them ran for their lives.
The Forretress are normally silent, but when an aggressor approaches, they speak in unison words understood by other pokemon as meaning "You shall not pass." Attempts to gain information from them have proven fruitless, save for one exception: when asked why they guard the walls, they answer that it is a "contract with master." Who this master is, or what they are guarding, is left unanswered and unknown.
Therefore, there are no reliable reports of what lies within the Forretress walls. The difficulty of flight within and thick canopy of the forest has made aerial observance impossible, especially as low-flying observers are within range of the Forretress' Gyro Balls. None have breached the wall in recorded history. And so mythology has filled the gap, although the myths do not agree. Some say that what they guard is a strange land where Celebi sleeps forever, even while simultaneously awake and messing with continuity elsewhere in a time travel paradox. Others say they guard the gate to hell, yet it is different to imagine any hell worse than an eternity as part of an unmoving wall, never speaking with your own voice. One can only wonder about their contract, and what it gave the Forretress to make it worth it to stay in this wall forever.