Legionnaire is a computer game for the Atari 8-bit series created by Chris Crawford in 1982, and released through Avalon Hill.
Recreating Julius Caesar's campaigns in a semi-historical setting, the player takes command of the Roman legions in battles against the barbarians.
Similar to the earlier groundbreaking Eastern Front (1941) in terms of display and general gameplay, Legionnaire added a real-time computer opponent, and is one of the earliest examples of a real-time tactics (RTT) game. The player took the side of the Romans, playing the role of Caesar, giving orders to the various legions represented in pink in real time. The computer played the barbarians, in blue. There were three types of units; infantry was represented by a sword, cavalry as a horse head, and Caesar's own Imperial Guard as an eagle. Orders were given to the units by moving a cursor over them with the joystick and then holding down the joystick button; existing commands would be displayed as a moving arrow, and new orders could be entered by pressing the joystick in the four cardinal directions.
Mid-game in Legionnaire. The Roman forces are moving into position on a hill (pink is the highest elevation), awaiting the arrival of the barbarian forces moving in from the right. Cicero's legion is selected, under the pink square cursor, and the display shows that it is in the process of moving to a new location, shown in blue.
Like Eastern Front, the Legionnaire playfield consists of a large grid of square cells with various terrain features displayed on it. Unlike Eastern Front the new map included altitude, displayed as a series of contour lines. Movement was effected by the contours as well as the underlying terrain, making positional combat an important part of the overall strategy. The screen showed only a small portion of the entire map at one time, smooth-scrolling around it when the joystick-controlled cursor reached the edges of the screen. Unlike Eastern Front, the map contained no cities or strategic locations, and the game started with both forces placed at random locations on the map.
While the user was entering the orders for their units, the computer was calculating moves for its own units. 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 player was forced to search the map for the enemy, and then attempt to gather their units into a fighting line on favorable terrain. The challenge was doing this quickly enough before the enemy forces arrived and attacked your units piecemeal. As in Eastern Front the AI was not particularly strong, but the real-time action made the game more difficult, as well as eliminating several "tricks" one could use to fool the AI.