An almost-solid shooter in sore need of editing.
You have the makings of a decent game here. The enemy variety for the most part is pretty cool, and I like some of the thought put into the upgrade system - the way so many upgrades branch off to benefit more advanced guns is nice, and gives us a reason to invest in some lower tech without feeling guilty for wasting the money. Moreso, later into the game I still feel like many of the earlier guns are still the most viable general-use weapons, although the bolt rod does somewhat start to replace them for a lot of general purpose situations once it's fully upgraded. My only serious issue with these is that it seems like a great number of enemies are immune to a wide variety of weapons, and that the immunities mostly seem handed out pretty arbitrarily. Other than fliers and crawlers being immune to certain weapons across the board, there are no consistent rules to remember and that's a -lot- of arbitrary special-case immunities to memorize once more monster types start unlocking. There are some balance issues with certain weapons, too, notably that the grenade launcher almost -never- seems worth the heat it causes on firing and doesn't even have a very large blast radius to make up for that, and that I find myself rarely ever using acid just because of its terrible shot speed. The grenade launcher might work better after unlocking the improved blast radius achievement, but that requires getting a lot of kills with this fairly useless weapon first and I'm just not up to that. Similarly, the option to hire a trio of gunmen didn't seem remotely worth the cost in "Risk level," and for how much of the gameplay otherwise hinges on upgrades, I honestly question why you implemented this "risk level" system in the first place. It really severely limits the player's option to take advantage of available upgrades. Getting back to the subject of achievements, though, between poor English and outright editing errors I honestly have no idea what half of these do. The game says I have 151 "bonus kills" with my machinegun but I can't even figure out what a "bonus kill" is so I can try getting more of them and finishing this off. The same achievement's label says it boosts bullet speed and in the description says it raises damage against normal zombies. Which is it? And why the hell did you let instructions this bad make it to your final submission? On top of that all, my achievements page credits me for getting kills with weapons I have not even purchased yet, which doesn't do much for my suspicions this entire system is arbitrary and buggy as hell.
