This is an image of the observation tower called Eagle Tower, which provides a 360º view of the surrounding state park. It replaced an earlier tower in the same location.
The description states:
Eagle Tower, Peninsula State Park — Door County — Wisconsin. One hundred ten (110) steps take one to the top platform of 75 foot Eagle Tower. A view from this point which is 255 feet above lake level is well worth the effort.
–The tower is still extant in this location, but has been closed to visitors due to its deteriorating condition.
This card likely dates from the 1950s.