Great!
This game was a lot of fun to play, all the times I played it. In fact, the reason I haven't written a review earlier is because every time I go here to write it, I end up playing instead.
The music is exactly the same as Age of War, which was a great game. It's evidence that not changing a winning formula makes you win again. The other sound effects are nice as well.
Good upgrade system, quite versatile on first sight. For first timers, the weapons aren't very balanced. I found it's easiest to focus on gun and arrow, leaving throwing weapons alone after the first few levels, because they are quite simply too inefficient and prone to missing. Shooting is just easier, and the ability to land KOs with just getting more powerful weapons, increasing your character's stats or getting some special effects into play is too valuable. However, I should note that I just did a run with throwing weapons only and I found that I could get to level 40 with it, and I could probably beat the game with it too. The final throwing weapon, with proper upgrading of Power, could OHKO everything for a long time. And once it stopped OHKOing, I had Double Pierce (I found Pierce much more helpful than Bounce, so I didn't go to Double Bounce), Light special, and a base that could survive two World Wars.
But, I also found the game getting quite laggy and unresponsive in the later stages when there's a lot on the screen, which was quite detrimental since aiming with a throwing weapon is hard enough already without the game not throwing at the proper time.
Speaking of things on the screen, I often managed to beat entire stages without anything ever entering the view. Apparently you can hit (or shoot) things even if they're not on the screen yet. I am not sure if this is intentional or worth fixing for you, but I thought I'd point it out. It helped me a lot, but I can see reasons for trying to remove that part since it isn't very...realistic?
Not a big fan of the ways some menus were handled. The instructions menu didn't tell me anything about what everything in the upgrade screen did, which isn't too great when the upgrade screen also doesn't do that. For things like Power, and new weapons, no explanation is needed. But I'm still not exactly sure what having more Speed accomplishes, for example, and you don't really get an idea of what the special weapons and effects do exactly without trying them out (I found the Special weapons very disappointing and unnecessary, personally).
The instructions menu is also a bit unnecessary because every single time you play a new game, the game forces it down your throat. You can ram through it with Next, but there's no option to skip it completely.
Personally, I think the best way to handle things like this is to have as much info available as possible in the Instructions menu, but if possible, to make the game playable without reading it (like having an optional tutorial at the start that teaches you how to throw and upgrade, etc). This way, beginners won't be confused, and experts won't be annoyed!
Last small nitpick: if P opens the Pause menu, I think it's best to also allow P to close it. Having to click "Close" after using P to open it isn't as intuitive.
Don't let all the possible improvements take anything away from the well deserved 8/10 score though, this game is very, very good.