Linking castle or communications castle: A castle playing the role of garrison, for contact and communication via smoke signals, for checking travelers crossing the border, and for gathering, transferring or accommodating soldiers.
繋の城・伝えの城　狼煙による通信連絡を行ったり、街道の関所、兵員の集結、移動、宿営のための駐屯基地などの役割を担う城。