Research: Now everyone has there ideas on what is the most important research things to concentrate on.  Early on I like to complete the research that is required to train troops as you are upgrading your court and being able to train new troops.  But over the coarse over the game you want to upgrade EVERYTHING to level 20.   Some things like Scouting can be upgraded to level 30.  Also you can upgrade your academy to level 30 as well.  Even thought it may not look important it definatly is.  It strengthens EVERYTHING!  Attack and Defense.  Some people who have the acadamy upgraded to level 27-28 have GREAT castles that are hard to beat.  Now probably the two most important and expensive researches are attack and defensive formations.  They are vital to having strong attacks and defense.  If you scout someone and they have attack and defensive formations and you do not it is probably not a good idea to attack them unless you know they are weak or have no troops.  Because more times than not you will lose!