battles in my opinion is quite fast passed. Sieges takes longer time a given as you need to pound the enemy's defense so its easier to get inside. I play Elven, Human and Orcs to get a nice feeling to them all.
And i prefer rush in open fields. i been quite successful on just that.
Elvens rush your sentrys up first then your sword infantry behind and long ranged rangers "all points on range to lvl 6" and when your bladestorms gets inside their lines use whirlwind and lots of enemy's dies. killed lots of crossbows and dwarven rifles that way.
and use your hero as a meat sheild as you can get 100% dmg resistance for a few seconds wich can also get into the enemy scirmish units and do alot of dmg
Orcs is i think the easiest race to just rush forward with. Goblin Raiders with max upgrades for their speciality's. and all level points added to dmg. wiill make them fast and very deadly and you get them for free in your tents. So you can easy make large armys and use your earned resources to heal them and build up your settlement and upgrade your units to lvl 5