Who takes design notes from microtransaction games? What's fun about having to hoard special shots from stage to stage? You added a metagame that says we have to try to beat each mission in the most boring way possible to make sure we have the resources to beat the harder ones.
Backpack seems to be about 5% per level, which really isn't much of an investment, later on. You'd have to earn a million - a hundred waves or so - to make up the cost.
It's like one of those platformers where the gameplay is wrecked by the tedium having to truck back and forth across the game world each time you find a key, except here there's no gameplay, just tedium.
4/5. Great, but there are quite a few levels with fake difficulty. Redoing the level several times to memorize deathtraps that can't be foreseen isn't fun. The Tunnel was a particularly bad example.
I'm sorry about that. We're already working on the text revision :)