My favourite army is Denmark.

5x Obudasher - a very very good unit that is way better for me then f.ex. Jannisary Heavy Infantry. They don't have a second weapon like pikes so won't switch to a crappy unit, and they can make a pike wall with their halberds. They stop a caalry charge and infantry charge with ease.
3x Norse Archers - medium infantry with arrow support, stand behind the Obudasher pepper the enemy with fire arrows, then support the pike wall in melee
3x Artillery (Trebuchet behind the pikes or Cannon on the side)
1x General
2x Huscarls or Chivlaric Knights - for flanking and pursue
6x Heavy Infantry - as Denmark you can choose different units, i usually use Dismounted Huscarls and Dismounted Feudal Knights

Defending a bridge or a wall assault is pretty the same for everyone so i will describe field tactics.
I choose a place near some woods i put the Obudasher in the open with Norse Archers behind and Trebuchets in the back. Cavalry a little behind them in a posittion to easily flank my pike wall and support it.
Heavy cavalry is hidden near in the woods. Usually the AI sends his cavalry after heavy infantry so i can pin them easily and finish them off. If they dont use a lot of cavalry i make quick fast attack from the side.