Do it in the browser so you can do it.
how do I castle in the android app?
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it's simply not possible in the app?
On the android tablet app i just move the king over 2 spaces. Thats the same as I do in a browser with a mouse. Instead of dropping it on the rook, try dropping the king on the square where it will land. Practice against the computer or in analysis mode. Also be sure you actually are able to legally castle (ie not in check or through check, etc).
this might be a silly question, but I can't seem to find a way to castle in the android app. just dropping the king on the rook doesn't seem to do anything - any suggestions?