I agree with much of what Mistermind says below. I think its a game with good potential, but some very odd design choices in some key areas that need looking at.
As Mistermind said, grinding is a bit broken. One needs to grind levels to advance; it simply isn't possible to advance, with all the gold of the previous level, to match the difficulty rise of the next level. But the levels with the hardest difficulty jumps, usually the ones with bosses, give very little gold. This leads to the unsatisfying and counterintuitive strategy of intentionally dying in levels you can beat, to collect their gold.
Furthermore, I don't find the advancement is well thought out. Its nice that the staff has to be properly managed, with extra runes causing shots to cost more mana, but there is no indication that it is happening, nor to what amount, so its a bit of a crapshoot. Further, I tried most of the runes, and only saw value in the damage-dealing ones. Speed runes seemed like they would be necessary, but didn't seem to improve the shot at all.
The special spells appear to have great importance, considering how many there are, and their prominence on the upgrade screen. But they have very little impact compared to similarly-priced upgrades, and there is no point in having more than one because they all come from the same pool. The mechanics of this need to be rethought.
Lastly, for some ridiculous reason, it appears you can't buy items from the shop that aren't available in later levels. So if you didn't shell out for the 800 gold rune-extension item when it became available, not only will you have to pay 1000 gold for the same bonus in the next shop, but you'll be limited to a total of 4 runes for the end of the game. That is, frankly, pointless. It's a curveball the player has no chance of anticipating unless she has already completed the game once, and while I didn't mind playing the game, I don't think it has any replay value.
Still, a fine enough game, with interesting retro graphics and music, and an okay difficulty curve.