The towers of Noah were specifically built for the purpose of overseeing the land for the incursion of Lamanite armies (see Mosiah 11:12). In this case, the tower does precisely what it was meant to do, and Noah sees the Lamanites in the field.
We may also speculate further on the conditions that lead to this particular incident. First, why would Noah run up a tower rather than away from the palace to open ground where he had more options. In this case he "treed" himself. Assuming that Noah was not completely stupid, he probably had few options. If Gideon effectively controlled the palace and the access to Noah, he would clearly control all of the exits. Therefore Noah's only hope would have been a defensible higher ground. For those who have seen the narrow and steep steps of Mesoamerican temples, one may see how such a "tower" might be a position of defense. With Noah on the flat top and Gideon attempting an uphill fight on narrow stairs, Noah's military position would be clear. In the conditions that appear to be described in the brief hints Mormon gives us, Noah probably did the only thing that had any chance of saving his life.