the best way that i found is to build three blasters one at each of the openings, that should keep the creeper at bay. then on the ledges between each cell, build one or two mortars on each cell. then build multiple blasters that will push back the creapers. the most important thing is getting the creepers stopped while you build the mortars
 "arkrueger"){kind=small}
 "Daniel973"){kind=small}