Recreating Julius Caesar's campaigns in a semi-historical setting, the player takes command of the Roman legions in real-time battles against the barbarians.

[citation needed] In the game, the player takes the side of the Romans, playing the role of Julius Caesar and giving orders to their forces in real time.

There are three types of units: infantry is represented by a sword, cavalry by a horse head, and Caesar's own Imperial Guard by an eagle.

[3] Like Eastern Front, the Legionnaire playfield consists of a large grid of square cells with various terrain features displayed on it.

A basic form of multitasking was implemented by having the "easy" jobs of reading the joystick and recording the user's inputs during the vertical blank interrupt (VBI), while the computer AI ran during non-interrupt time.

The game lacks any strategic component driving the gameplay, so it is possible for the player to simply move to an advantageous position, form up a solid line, and wait.

In pursuit of a higher score the player is forced to seek out and attack the barbarian units on terms that might not be as favorable and which, without quick retreats after combat, might result in encirclement or exhaustion.

"[3] Softline called Legionnaire "a well-documented and thought-out program ... easy to learn and difficult to play well", with graphics that were "a fine piece of work", and concluded that "On a scale of 1 to 100, this is a 95".

"[11] BYTE mocked Legionnaire's cover art as being of poor quality, but called the game "great entertainment, even for those who haven't been war-game fans before".

[13] By contrast, BYTE columnist Jerry Pournelle, a fan of Avalon Hill board games, disliked Legionnaire.