Fortified Manors (居館, kyokan) are the fortified residences of powerful local leaders and samurai. They are typically simple rectangular fortifications with a moat and earthen embankment, or sometimes stone walls. In Japanese they are often called Yashiki or Yakata. Many of these Fortified Manors are from the Heian and Kamakura Periods, before castles enlarged and developed to include quarters for samurai and other powerful local lords. In the Sengoku Period you often see the pair of a fortified manor at the foot of the mountain for everyday use and a mountaintop castle for use in times of unrest. Kazurayama Castle is one such good example and so is the Sanada Palace / Sanadahonjo Castle pair.
Some might ask if such Fortified Manors are really castles. The definition of "castle" in Japanese is actually vague. It is simply a fortification to help protect from one's enemies. This includes everything from the earliest fortified villages (Yoshinogari) to primarily political centers (Taga Castle) to smoke signal towers (noroshidai) or even fortified towns like Kamakura which itself was called Kamakura Castle at one point. The military purpose and defensive fortifications are what separates these from ordinary homes. This distinction is also separates them from Samurai Homes of the Edo Period, which were strictly controlled by local lords and did not have castle-like defenses such as embankments or trenches.There is a frequently used Japanese castle term 城館 (jokan), which includes the characters for both castle (城) and fortified manors (館) that clearly covers both. There are also a few yakata in the Top 100 Castles and Next 100 Castles.Flatland Castles. The much more accessible site provides the lord with a more suitable location for administration and the ability to develop a castle town surrounding the castle, which itself can act as a defensive perimeter. Iyo Matsuyama Castle and Himeji Castle are two exemplary original castles which display all these features. The Hilltop Castle is also known for reforming the land and carving the hill to meet the purposes of the castle, much more significantly than a Mountaintop Castle likely would have.
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.