The Collaborative International Dictionary of English v.0.48:
Steeply \Steep"ly\ (st[=e]p"l[y^]), adv. In a steep manner; with steepness; with precipitous declivity. [1913 Webster]WordNet (r) 3.0 (2006):
steeply adv 1: in a steep manner; "the street rose steeply up to the castle"