The gameplay takes on a 3d-esque isometric view, in which the player chooses one of four mercenaries to lead through the levels. The characters act as lives, and if one were killed, the player still has the choice among the remaining mercenaries (rather than having multiple lives for one character). In the levels, the player must find keycards, destroy guards, aliens, and robots, activate laser bridges, to complete an objective. Objectives include finding the exit to the level, assassination of an officer, escorting a defect or other important official, retrieving an item (usually a memory chip or the like), or obtaining a certain percentage of kills in the area.
The four characters available to the player are for the most-part alike, but each have key differences that make them noticeably set apart from the rest. For example, the large alien, Quogg, has a melee attack that will kill the standard human troops in just one hit (rather than the two required from the other characters). Also, each character has slight variations in weaponry that make for somewhat different combat experiences.
Source: Wikipedia, "Project Overkill," available under the CC-BY-SA License.