Possible solution for place mutation (somewhat functional approach)

gistfile1.txt

# in board initialize
lookup = PlaceLookup.new
lookup.add Buildable.new, Coord.new(3, 4)  # coords would be calculated from land hexes

# later
player = Player.new
PlaceLookup.build_at player, Coord.new(3, 4)
PlaceLookup.at Coord.new(3, 4) # => Settlement.new

The PlaceLookup mutates behind the scenes.  So potential impl of #build_at is
class PlaceLookup
  def build_at player, coord
    # when you call build on a Buildable object you get a Settlement.new back
    @places[coord] = @places[coord].build player
   end
end