playing this, I can say that as far as touch screen games go, this is the right kind of game.
If you're too emotionally invested in the original style of games, then this won't work for you. Touch screens can't offer the same gameplay for fighting games, or even run n gun style games. Shooters on touch screen devices suck too. If you want to argue, that's your prerogative. But as I see it, touch screen devices lend themselves to a different style of game.
Metal Slug Defense is a simple game.
You don't need to spend a penny on it. You can spend money if you don't want to unlock things naturally. You can have that option.
The "ammo" replenished itself over time, much like credits replenish in the iOS Arkham game that came out last year. So if you run out of ammo, just do something else. Play a different game.
There is an RPG element in that you can upgrade your units or your base. You start out with a weak grunt, who can either toss grenades or knife enemies. You can also set down sandbags to shield you from enemies. After you clear some stages, you can use Marco. Clear the next level and you get Tarma. Then Fio, then Eri, etc.
Anyhow, I highly recommend the download.