The location really is perfect for keeping an eye on the entire surrounding area. The ports and even Honshu (the main island) are visible to the north on clear days, while the valley surrounding the castle affords views for miles in the other directions.
Kameyama Park is the official name of the park with Marugame Castle, and it's located about 15 minutes on foot from Marugame Station. You can catch a bus for 200 yen that will get you there in 5 minutes. The grounds and museum are free, but entrance to the keep will cost 200 yen - not a bad price to pay for an original castle. The keep is open 9:00-16:30, closed the last week of the year.