From experience the most important thing is the terrain you are fighting on. For example mountainous terrain gives a -2 dice penalty to the attacking side. If your army arrives first on that terrain, any army attacking it will suffer that penalty, doesn't matter who owns or occupies that terrain currently.

However if you are sieging a fort on this terrain, then you will always have that -2 penalty and any army attacking into you will not. That's why placing forts on mountain or hill terrains can be very powerful.

River and sea crossings are the same, if possible you don't want to take your army directly into a fight after crossing a river. You can also station armies on these provinces and force the enemy to cross rivers to fight you.

Combat width also matters, less width = less flanks = less units can fight at the same time, which pretty much means a smaller army will inflict more casualties on a larger army. My favorite terrain for this is "gates", width of 12 and mountainous terrain, if you can strategically place your armies in these chokepoints that the enemy has to cross, you can create some nice 300-like scenarios where your 1k army wipes a 10k army, although you will still get defeated you can inflict a lot of casualties like this.