Thanks for the feedback, and glad you enjoyed it!
Using the jump button in addition to crouching to go down one-way platform is an interesting idea, I will consider it. ;)
My engine is modular, so while I do have a module dedicated to metroidvanias, I've also built a Zelda-like, a JRPG and even some 3D games with it. I plan to write up a detailed post-mortem at a later date, explaining my process and describing my tools, but in the meantime I do talk a bit about the engine here if you're interested.