You need just 3 stones for the cannon.
The last one is in the mining town (the first location after you leave the laboratory at the start), speak with a blue shadow near the bridge, it will exchange a stone for some resources.
The resource you need is going to be either the Snake Fang, which you find on a giant snake (level 25) near the South Army Base, on the desert map, or the Diamond, which you get from the Level 20 Minotaur near the road to your home.
I'm not sure which one it was, but it was one of these. You also need them (several snake fangs, in fact) to get the best weapons in the game.
As game 4/5 - the balance, UI, and level design are not perfect, but the game is short and ends before the problems become annoying.
As porno game 1-2/5 - scenes are very short and unnecessarily split into parts, and animations are bad.
Only one ending.
Yeah, everyone's talking about "wasted potential", and it sort of is , it could have easily had 3x more scenes and more character development, the third girl gets no scenes, second girl barely has anything in both R18 scenes and actual development...
The story setting, maps, battle system, character design, audio, voice acting, everything is fine, better than most games out there, it's just that it feels like the developer forgot somewhere along the way
that this was supposed to be a porn game, and when he remembered, he cramped 2/3 of the R18 scenes of the game right at the end of it.
This could have been one of the best R18 JRPGs. The ingredients were there. But the chef forgot to cook half of them.