I own a condo with a small backyard held up by a retaining wall at the very back part of it that goes down about 3 feet and just behind it is my neighbors fence about 18 inches. So there is this pit of death sitting between the retaining wall and the fence.... I have little ones and want to know what can I do to either cover this up or build over it or fill it in without ruining the fence. Any ideas? P.S. the retaining wall is made out of large boulders and fence is wood.