It filled a valley much larger than its sister keep's, widened farther by quarry-work which had left sheer, surrounding walls of live granite. […] Wilden itself angled down the valley's floor, presenting tier after jagged tier of its internal structure, tower after terrace after tower. Between the trapezoidal jaws of its double outer walls, it looked like a mouth full of sharp, ragged teeth. […] Through [the doors of the upper gatehouse], white glimmered in the premature twilight: the ruins of the hill fort around which Wilden had been built, carefully preserved on a tiny hillock in an inner courtyard like white bones cradled in Randir jaws.
– Jame's narration, Seeker's Mask, "Part III: The Riverland", VII
Several streams follow down the valley Wilden occupies, and the keep has taken advantage of these. There's a waterfall at the upper send of the valley, which flows around either side of the keep. As it follows along the sides, more streams join it. Before the outer walls, they streams are damned, making a moat.
↑Seeker's Mask, "Part III: The Riverland", VII — "a considerable waterfall at the valley's upper end. Streams plunged down on either side of Wilden between the mountains' granite flanks and the inner walls of the fortress. […] The streams had been dammed before it to form a brimming moat. From this, runoff driveled down the steep outer ward toward the curtain wall that sealed off the valley, toward the River Road and the Silver below."