Absolutly brilliant. The only game better is Rome Total War.
In Medieval 2 Total War, like the other Total War series, you can either play the Historical Battles (eg Battle of Hastings), custom battles or the campaign. For the custom battles. You can select from numerous maps and then build your army. There are so many different factions such as England, Papal States, France, The Holy Roman Empire, Moors and so on. Each faction usually has a number of unique units like the English get English Knights, the Papacy get the Holy Cross and the Hungarians get the Hungarian Hussars.
For the campaign, Medieval 2 Total War is a masterpiece. The only campaign I have enjoyed more has been Rome Total War. The Medieval 2 Total War campaign is good for beginners and veterans alike. As the player progresses through the campaign, declaring crusades, conquering nations and expanding settlements, they unlock factions. Some factions are not playable however such as the Papal States but you can always edit the files which is very easy. (Search on Youtube for Medieval 2 Total War unlock factions)
A brilliant feature of Medieval 2 Total War, which Rome Total War does not have, is religion. There are, basically, Roman Catholic Factions, Heretic Rebels and Islamic Factions. If, for instance, you play as the Catholic English, you need to increase your favour with the pope by building chapels and abbeys, spreading the Christian faith and being relatively peaceful with other Catholic states. Eventually you will be able to call a crusade on a non Christian settlement. However, if your favour with the Pope and the Papal Church drops right down, you will be excommunicated and don't be surprised if a crusade is called against you.
For all the Total War series, the music is beautiful. If you like good, slow, harmonic but also epic and ambitious music, play Medieval 2 Total War Kingdoms. The soundtrack for Medieval 2 Total War is 10/10.
To conclude, the only game better than Medieval 2 Total War is Rome Total War which is, basically, the greatest strategy turn based campaign ever made. I think Medieval 2 Total War is slightly harder than Rome Total War and there are less historical battles and just less of that 'game feeling'. For the gameplay, campaign and soundtrack, Medieval 2 Total war is revolutionary. - No Total War series is complete without Medieval 2 Total War.