This is a tiny little castle. But its very worth visiting. We went in summer, but there are lots of cherry trees so its sure to be good in spring and the whole area must be lovely in autumn too. You can combine a trip here with a trip to Nanko Park which is only a very short distance from here.
You can take the train to Shirakawa and walk to the castle and then Nanko park would just be a short taxi trip. The drive north along highway 294 is quite beautiful though I'm sure the freeway is faster.
In June when were at the castle the rose garden was also in full swing. Nestled under a castle wall is a very attractive location for such a garden.