I'm building it from scratch in C++14 with SDL, blitting every texture like it's 1995. I may migrate to OpenGL eventually, depending on how well the blit-krieg approach scales. We'll simply have to see about that.
I'll attach some relatively recent video of how the game looks today. This is from an experiment with a hybrid maze and cellular automata algorithm I've been working on. It's still a work in progress, but I'll try to write more once I work out some of the kinks.
As will be abundantly clear to anyone who has played Commander Keen, that is my main stylistic inspiration, although combat-wise it's probably closer to Duke Nukem 2. In summary, I really like ID Software's old pre-DooM creations, and that whole shareware age aesthetic in general :-)
At this point, the game design is fairly undecided. I like the idea of a Terraria-style procedurally generated world, but I want more emphasis on combat and none of that crafting nonsense. More Borderlands, less Minecraft. I'm also looking for a way to negotiate a procedurally generated map. Having the whole Flappy Bird air-jumping I enabled in the first video looks a bit silly.
I'm also drawing all of the assets myself, which is arguably a bigger challenge than the actual programming at this point, although I'm slowly improving in my spriting abilities.