The Lanteans designated this world as a "safe haven" for anyone wishing to avoid the Wraith and devote their lives of meditation to learn how to ascend. A massive portion of the planet's surface is a time dilation field. Overtime, the field caused a crater, as mountains were formed on the edges of the area. Time inside the field passes around 250 times as fast as it does outside it. Even if the Wraith were aware of the planet, there is a layer of shielding, so any ships or objects sent down would be unable to penetrate the field from orbit, and would be destroyed. The inside of the field also contains its own day/night cycles, as well as artificial weather.
Lt. Colonel John Sheppard and his team once arrived on the planet to explore it, but Sheppard was sucked into the barrier and spent, in his own perception, a total of six months within. (SGA: "Epiphany")