|Pre-Edo Period and Edo Period descriptions
This category includes all those castles that were active in the Edo Period. At the beginning of the Edo Period the "One Castle Per Province" edict forced the abandonment of thousands of smaller castles throughout Japan. The castles that remained into the Edo Period or were newly built in the Edo Period saw many changes to their construction compared to earlier castles. Since a lord could only have one castle, it tended to be bigger. Castles also shifted to Hilltops or Plains from the mountains to make them more accessible and easier to govern from. As techniques improved you would see also bigger stone walls, wider moats to prevent gunfire, and larger main keeps. Some of these techniques were developed at the end of the Sengoku Period in places like Azuchi Castle and Osaka Castle but they really became most widespread in the Edo Period to the point where you could say they were standard features of Edo Period castles.
Edo Period castles also includes a subset of fortifications called jin'ya. Jin'ya are basically fortified government offices. They were built for small domains (generally less than 30,000 koku of rice) where the lord did not have the qualifications to be a "castle lord." These were called "castleless lords" or "jin'ya lords". Jin'ya were also established for domains, like Takayama (see Takayama Jin'ya), that were directly under control of the Tokugawa government. Jin'ya are often listed as castles in castle books and materials.There were 300 provinces (with some minor changes through the period) in the Edo Period, so you could say that in the Edo Period alone there were 300 active castles (including a handful of jin'ya). For people who are new to visiting Japanese castles or whose image of Japanese castles is Himeji Castle these Edo Period castles are a good place to start on this adventure. With a few exceptions, all castles labelled as Edo Period in here are mostly cataloged in these books about the Edo Period castles of the 300 Han Castles and Jin'ya of 300 Edo Period Domains - East Japan (図説江戸三百藩「城と陣屋 (東国編)) and Castles and Jin'ya of 300 Edo Period Domains - West Japan (図説江戸三百藩「城と陣屋」(西国編)).
Castles that were only active prior to the Edo Period come from a very different time of castle construction. The Edo Period saw the introduction of the "One Castle per Province" edict which forced the abandonment of thousands of castles throughout Japan. Castles prior to the Edo Period tended to be a part of a wider castle network. There would have been a central home castle where the provincial lord may lived and ruled and there was also a large network of small castles that would protect the borders, watch over major roads, serve as smoke signal forts, or may have simply been created and generally left empty except when needed in a time of unrest.
Any castle with this designation is probably smaller in scale, often on a mountaintop, and is unlikely to have any buildings. These sites will be of more interest to history fans, castle fans, and those who want to understand a specific area better.
This category also includes some of the ancient castles such as the Josaku sites ((e.g. Shiwa Castle, Taga Castle)) in Northeastern Japan and the early fortifications in Western Japan such as those at Kinojo and Ono Castle. I will likely separate these out into other categories at some point.