Sonic Jam's Level Design Guide http://www.smile.talktalk.net/sj_index/nss/nss_contents/index.htm
Unlimited Trees' Level Design Guide http://sonicresearch.org/community/index.php?threads/trees-terrible-guide-to-sonic-level-design.4797/
Sonic Retro Level design guide http://forums.sonicretro.org/index.php?showtopic=28302
Hirokazu Yasuhara's "Fun" Guide https://www.youtube.com/watch?v=J-3avMBqJ9s
AM's How Controls Define a Game https://youtu.be/crghQhtc_jU
Mark Brown's Putting Play First https://youtu.be/2u6HTG8LuXQ
Extra Credit's Speed-run Guide https://youtu.be/pm8KdLPOm2c
SnomaN's Speed-run Design Guide https://youtu.be/AecPaV5GBIk
Tom Hall's level design notes (Part 1) https://3drealms.com/news/3d-realms-vault-1994-design-tips-tom-hall-part-1/
(Part 2) https://3drealms.com/news/3d-realms-vault-1994-design-tips-tom-hall-part-2/
Tim Ryan's "Beginning Level Design" (Part 1) http://www.gamasutra.com/view/feature/131736/beginning_level_design_part_1.php
(Part 2) https://www.gamasutra.com/view/feature/131739/beginning_level_design_part_2_.php
Gamasutra's Sonic Level Design Analysis http://www.gamasutra.com/view/news/117691/Analysis_Sonics_Game_Design_Influence.php
Glixel's Sonic Mania Team Interview http://www.rollingstone.com/glixel/features/a-chat-with-fan-turned-creator-of-sonic-mania-w501581
Gamasutra's Mario 3 Analysis (Part 1) https://www.gamasutra.com/blogs/RadekKoncewicz/20110225/7089/Super_Mario_Bros_3_Level_Design_Lessons_Part_1.php
Useful GML tutorial: http://www.wou.edu/las/cs/csclasses/cs125/tutorials/GMLTutorialv6.pdf