Castle ruins are places where there was once a castle.

In Kani City there are as many as 10 locations that have been identified as medieval castle ruins. As many of their historical records were composed in later ages, the details of the ruins are still unclear. However, through using demarcation research to analyse the construction of the castles, it has became apparent that many of them were fortresses which were used during the Battle of Komaki and Nagakute in 1584.