Needless to say, most popular season is autumn, in which you can observe a sea of clouds. But, not only in autumn, you can enjoy Takeda castle in other season.
There are cherry blossoms around Takeda castle including Ritsuunnkyo.
Autumn(September to October) :
Takeda castle looks floating in a sea of cloud.
It is very hot, but you can enjoy blue sky with takeda castle.
Takeda castle covered with snow is also very beautiful. You can go Ritsuunnkyo during closed period for takeda castle. But because of snowy road, even you have snow chain on your car, still difficult to get there.