I was thinking about your original post, my reply and such. I was wondering about the rendering of the camera and such. Despite the rendering being set to a particular size, there is still the problem of the overall upscaling to a viewport that makes the display usable in any fashion. It seems that most entries set the display canvas to be something like 512x512. I went with that. That leads to the rendering engine to scale from 64 to 512. In that, the integrity of the single pixel will certainly be lost.
I was wondering about changing the unit pixel size to be something larger (say 10) so you can maintain an overall logical pixel size of 64. Given a 10x10 display pixels to be a single logical pixel, you could render the game at 640x640 with a single unit being an even chunk of that canvas. The author would then be entirely in control w/o the rendering engine needing to scale at all. Reading your last reply, this is where you have ended up yourself, it seems.
Anyhow, it is interesting to ponder.
P.S., swing the sword with a left-mouse click (which I see I totally omitted from the controls section).
LibreSprite - I'll check it out. I was also thinking to fork and compile Aseprite. I didn't mind supporting them, though. It is a pretty nice tool.