Because of the entrance's size, the Wardens feared darkspawn would find a way through despite their efforts. Senior Warden Valeska recommended they build a fort over the collapsed tunnel, so Wardens could always guard it; they did so for centuries.
The Grey Wardens' disappearance means Valeska's Watch is now unguarded. If the seal over the entrance has degraded, darkspawn may infest it. Be wary when entering this area. The blight is a slow death, and one I would save you all from suffering.