Ah the extra lives; if you press the space bar, you trade in some of the gold that you've banked in the middle of the screen so far for an extra life. The value of an extra life goes up every time you buy one though, so the idea is to only buy if you're really desperate, because "every life matters" and all that jazz. It's explained in the download page description and it's repeated in a little "How To Play" text doc in the download zip itself, though I can always give it a bit of a rewrite for clarity's sake if you want.
Glad you enjoyed it though! 🤠