u/Certain-Objective307

Android, free, pre-2017, portrait mode, top-down fixed camera

Single stationary character behind upgradeable barrier (sandbag → concrete with spikes)

Character rotates when aiming, 3 zombie spawn points at top of screen

Zombie types: normal, fat/exploding, dogs, enraged/red

Weapon upgrades: RPG, golden minigun

Colored special abilities: blue = freeze, yellow = terminator wipe, red = fire

Dark gritty art style, game icon possibly had a gas mask character

Name possibly starting with "un"