Gettysburg is an ultra-realistic, historically accurate game set during the the Battle of Gettysburg, with a real GIS capability map including terrain data, ultra-realistic tactics that take into account the opposing unit's position, active troops, overall strength, fatigue, supply, morale, and the unit commander's command and control abilities. Fight the Battle of Gettysburg as the Army of Northern Virginia under the command of Robert E. Lee, or as the Army of the Potomac under the command of George G. Meade. The computer will play a historically accurate opposition. Will decisions that you make alter the outcome of the battle? Can the Army of Northern Virginia prevail?
This is the closest I've yet found to a computer version of the classic Avalon Hill war games of my childhood. In fact, it appears to be virtually identical to the AH game of the same name, but with ability to simulate movement and combat in real time. (Although the ability to stop and start the clock also allows you to simulate turn-based play, if that's your thing.) Computerization also allows for more realistic play by incorporating factors such as supply, attrition, and fatigue into unit capabilities. Ability to rotate unit heading (for infantry) and aim (artillery) more accurately reflects 19th century battlefield tactics, especially the use of flanking movements.
Terrain map is primitive and elevations and coordinates are not clearly marked. Inability to zoom in and out of map makes it hard to separate and precisely position units. Skills of opposing AI general are pretty weak. Game lacks "what if" historical scenarios (i.e. what if Army of Potomac main force had shown up on Day 1, what if Stuart had been around and Lee had been able to concentrate sooner, etc.) No Windows version, as far as I can tell.