At the start of the battle you deploy your troops and then the fight begins with your army clashing with another. When an army attacks another army or settlement a battle commences.
Diplomats are made to announce alliances or trade rights, spies give information on armies and settlements, assassins kill characters like generals, and priests/imams spread their countries faith throughout lands and denounce heretics. There are agents in the game like diplomats, spies, assassins, and priests/imams. Differentiation of soldiers is also in the game.
One very cool thing is that when you upgrade a soldiers armor it actually shows a different image on the battlefield than its original stage.
You don't have to do any of the missions, but if you deny the Pope he won't favor your faction and inquisitors will sweep through your lands and trial your generals and you may lose them if you don't have enough piety. You will also receive missions to do from the Pope or the Council of Nobles. You must build necessary buildings in order to recruit special things or gain bonuses like barracks are for recruiting infantry units and farms give a better food production letting cities survive longer in a siege. Castles produce military units, less economy, and have less population. Cities are for producing more money not military and have higher populations. You can adjust a tax rate in a city, but not castle. There is only a source of money, no food and building supplies like other strategy games. You don't have to leave them like so, you can convert it to the other. In the beginning you start out with a number provinces with some as a city or castle. The game spans from 1080 to 1530 so there's 250 so turns. The game commences in turns and each turn is 2 years. Each faction has its strengths and weaknesses and units from splendidly armored French Knights to Spanish Conquistadors. You start out with a couple of factions first, but you must destroy other factions to unlock them. In the game Medieval II: Total War, you must lead 1 of 17 kingdoms to total domination using trade, religion, espionage, alliances, and military might to achieve this glorious victory.