Mountaintop Castles were the predominant castle type prior to the Edo Period and most common during the Sengoku Period. During the more peaceful times they were often used as a lookout point or relay point for smoke signals and to watch major roads into one's territory. People typically did not live in Sengoku Period mountaintop castles. Logistically they were not well suitable for governing or living so lord's would often have a fortified home in a more reasonable location with one or more nearby mountaintop castles where the lord and his family and retainers could flee to during times of unrest. The common features of mountaintop castles are trenches and ditches of various types used to impede the movement of attackers and create lines of fire to more easily shoot arrows at the attackers. Mountaintop Castles may have also used some earthen embankments to create barriers around baileys and they would have also carved the mountainsides to make them more steep (kirigishi). Stonework was mostly used to shore up the sides of baileys to create more open spaces and prevent erosion until the castles of the latter Sengoku Period when Oda Nobunaga and Toyotomi Hideyoshi started building mountain castles with much more stonework. Their power struggles mostly focused on central and western so that is where you see the most Sengoku Period mountaintop castle stonework. The Nagano area also has some fascinating stonework at mountaintop castles, that developed independently from the Nobunaga-Hideyoshi influence.
Mountaintop
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Hanagoya Castle (Murakami Clan Fort Network)
Iizuna Castle (Murakami Clan Fort Network)
Kamei Castle (Murakami Clan Fort Network)
Kashiwayama Castle (Murakami Clan Fort Network)
Kemurino Castle (Murakami Clan Fort Network)
Kokuzousan Castle (Murakami Clan Fort Network)
Komaruyama Fort (Murakami Clan Fort Network)
Mochikoshi Castle (Murakami Clan Fort Network)
Monomi Castle (Murakami Clan Fort Network)
Takatsuya Castle (Murakami Clan Fort Network)
Torikoya Castle (Murakami Clan Fort Network)
Toyo Castle (Murakami Clan Fort Network)
Tsubame Castle (Murakami Clan Fort Network)
Tsumi Castle (Murakami Clan Fort Network)
Ushibuse Castle (Murakami Clan Fort Network)
Wagou Castle (Murakami Clan Fort Network)