The Divine Court shattered the fortress of Athran Kir in a dozen pieces. Some of the larger shards of the redoubt remain interred among the many under dark caverns and other smaller features, such as doors, windows, columns, etc., litter the caves as well. The deciding location of the more intact segments is entirely within your discretion. This product details the approach from the surface and four separate intact sections. Many, many surface holes reasonably exist to provide ingress, so getting the PCs to the caverns map is easy.
You may choose to place the surface hole in an area of your world suitable for the former location of the Athran Kir. If so, choose a large forest that borders a mountain range. Where the forest meets the mountains serves as the most natural choice for the former keep. In that case, a surface hole some where in the vicinity leads to the caverns map. Otherwise, simply progress to any of the four entrances on the caverns map whenever you feel the PCs have traversed an adequate amount of the “wild”.