Mizu-zuka (a mound used as a shelter) was a stockroom for food in times of emergency from a different perspective, and mikkyo-zuka (mound of esoteric Buddhism) was basically an accumulation of residues much like kai-zuka although it has some religious implication.
水塚も見方を変えれば、緊急時のための食料を集積した貯蔵庫であるし、密教塚も宗教的な意味合いもあるが、貝塚と同様に残滓の集積でもある。