Now to understand an ambush, you need to know that the strategic map has been completely changed. It's now fully 3D, and the forests, mountains, hills, rivers and roads you see on it will appear in your battle maps. Units like armies, diplomats and spies have limited freedom of movement, depending on terrain, on this map. Each unit has a set number of movement points and you are no longer guaranteed to cross an entire province in one turn, never mind get to the enemy city. Not only that, but armies can lie in ambush on the map, stopping their movement early and preparing an attack on their enemies. Should you blindly set a long marching order into unknown ground, it's quite possible to stumble into one of these surprise attacks.
An ambush is a nightmare of a tactical battle. The enemy starts off prepared to the player's forces and very close. There's no possibility of deployment and, to make matters worse, the player's formations are in marching columns rather than battle order. Thus, they are long, thin, facing the wrong way and with their flanks completely exposed. Combine this with the fast pace of combat in Rome, and you are truly lucky to survive one of these attacks.