Protector is a tile-based tower defense strategy game. It is the first game in the Protector series. The series inspired a spin-off called Little Protectors, which is also a tower defense game.
The monsters in the game follow a pre-defined path and the goal of the game is to prevent the monsters from getting through the path. In order to kill the monsters, you can recruit five different units by spending gold. Each if them do a different elemental damage and has a special effect on the monsters. Fire mages do Splash damage to multiple monsters, Frost mages slow down monsters, Poison mages does damage over time, Energy mages give the monsters an electrocution de-buff which reduces their elemental resistances and Dexers have a high chance of critical hits but cannot hit flying monsters. There are four advanced units that has to be unlocked buy spending skill points. Each monster type in the game has resistances and weaknesses to certain elements. Some might even absorb damage from an element and gain life from it.
When the units shoot at monsters, they gain xp. Once a unit has enough xp to gain a level, there's a gold cost to let that unit level up. On each level, there are two upgrade paths, Specialist and Generalist. A Specialist upgrade gives a higher boost to the unit's special skill and a Generalist upgrade gives a higher damage boost.
There are 23 levels in the game with their own number of waves. Beating levels reward you with skill points which you can spend upgrading your skills. There are five lines of upgrades which each improves one of the five basic unit types. Four different upgrades are for unlocking one of the four advanced unit types, Alchemist, Necromancer, Archer and Chiv Dexer. The last four upgrades reduces gold costs, gives you more lives and increases xp gained.