The placeholder tile images floor.png and cwall.png are from the public domain
roguelike tileset "RLTiles". You can find the original tileset at:
http://rltiles.sf.net
+
+Wall and floor tiles:
+https://commons.wikimedia.org/wiki/File:Concrete_aggregate_grinding.JPG
+https://commons.wikimedia.org/wiki/File:Concrete_Casting_Defect_Blowhole.jpg