Because a tenshu was a purely military construction and at the time Japan was in the middle of the Edo period, an era of peace, the tower was never rebuilt. All that is left today is the elevated stone foundation, a place arranged today to serve as an observatory for the surrounding garden. Here's a photo taken from the former main tower foundation, with the Nijō Honmaru garden and palace.
EXIF info:
Nikon Df
Lens: 24-70mm F/2.8G
Focal Length: 32mm
Aperture: F/6.3
Shutter Speed: 1/800s
ISO Sensitivity: ISO 800