-property :extension, String
-property :max_zoom, Fixnum, :default => 23
+property :extension, String, :default => "png"
+property :max_zoom, Integer, :default => 18
+property :url_aliases, [String, Array], :default => []
+property :revision, Integer, :default => 0
+property :overlay, [TrueClass, FalseClass], :default => false
+property :default_layer, [TrueClass, FalseClass], :default => false