Good question. This was the first game I published 7 years ago. I made it with GameMakerLite (vers. 8.1) and used a lot of free assets, for instance, to quote my game credits:
"Most of my assets came from GameMaker (www.yoyogames.com/resources). I'd like to thank also: www.freesound.org for the music and sound effects, particularly ( "Gully with water drips" by Hintringer; "Tracked vehicle" by Cognito Perceptu; "Laugh_old_man_6" by Adam_N; "ClockTicks" by Daveincamas; "Marching" by WebbFilmsUK; "owl.wav" by Johnc). Many thanks also to www.opengameart.org particularly for ("Dungeon Crawl 32x32", AAVV, by Chris Hamons; the original tileset is at http://code.google.com/p/crawl-tiles/downloads/list"). I'd like to thank the artists: Redshrike e Thompsonj per "Golem LPC", "Fenrir wolf" (leeor-net), "orologio a pendolo", Tiles for Doc's garden. Many thanks also to: Andrew Garcell for his "Sitting Skeleton"; Raidensan "Rain"; Cyanowl "Shabby Chic Exterior Tiles"; Naarshakta "Puzzle pipe set"; Bleed "Tree collection"; Qubodup "Pink Helicopter"; Hyptosis, Zabin, Daniel Cook "Castle Tiles for RPG's"; Zabin "Wizardtower" oil painting; Lanea Zimmermann Sharm "house inside" e "clock animation" and many others). This was my first game and it will always be free for the help I had from you guys!"