When a unit breaks you no longer control it, it will run and eventually rally and regroup so you can issue new orders. Units can also surrender. This means they break and run and try to escape the map. If they do, they live to fight another day, but they are lost to you for that battle. In addition to ranged weapons like crossbows and bows you have short range fighters and mages.
Mages are the most powerful fighters, but are rare.
They can cast magic spells to help and harm. The repertoire of spells is very limited as are their ranges. But, magic has its own limits. Only so many points of spells may be cast in a given time period. Magic items have their own uses.