When you see the upcoming battle, first have a look at the Calradia map; the battle will take place in the exact location where the parties involved are at the moment. So - if it is in the open field, the battle will take place in the open field. If the attacker conquers the village, the battle will take place in that village, if town or castle - you will have a siege.
You need to apply for the battle, and wait until you are accepted. It may never happen. Sometimes (in big battles) there is more applicants than places in roster.
Cav is very useful indeed. Very expensive as well. And you need to remember that the parties need to provide equipment to all troops i.e. if you have 1000 vs 1000 battle, each party have to buy eq for 1000 warriors. 1000 boots, 1000 armour, weapons etc. For that reason there are battles with or without cav, depending on terrain, but the principal decision is made by commanders who buy equip taking into consideration budget they have.
About builds - some players think about Strategus when deciding on class they want to play. I fully agree with Elleryie and Braeden - make a build that suits you, just to have fun playing cRPG. Apply for Strat battles and try to do your best. Join the clan!