So yeah, what I said. It is always better to be defending, thanks to terrain bonuses.
No, that's not what I'm saying. If your troop types are strong on offense and weak on defense (like the aforementioned Gallowglaigh), you're better off dodging an attack (by moving to an adjacent province), then counter-attacking the enemy (preferably in open terrain where they won't get the bonuses).
If you get attacked and you have Gallowglaigh Infantry, your troop stats for
the entire combat are 0/0/1, which is pretty awful. You're better off moving to a different province to avoid the attack, then returning to attack the enemy stack so your troops stats are 0/2/4.
Combat doesn't switch back and forth between each side getting an attack and a defend. One side is the attacker for the entire combat (the side which moved into the province) And the other side is the defender of the entire combat (the side which was already present). So if you have offensive troops and get caught on defense, you're in trouble. Likewise, if you have defensive troops and initiate an attack, you're going to fare poorly without overwhelming numbers or a big difference in morale. BUT...you'll excel at repelling attacks from larger forces.
Terrain bonuses weaken the attacker's strength, but they're not a cure-all. (Unless the enemy is overwhelmingly cavalry and is attacking you in mountains, in which case you're sitting pretty unless he brought Tamerlane along).
If your forces are slanted to the offensive side, your best strategy is to attack enemy armies and pursue the defeated remnants mercilessly until you can obliterate them. Then you can siege at your leisure and send relatively small forces to any province where the enemy starts to build new armies, so that you can obliterate them the moment they're done. Wars will be relatively short but bloody. This is why a high Offensive rating is good, because your leaders will get a higher Shock rating, and do more damage to the enemy when in the field (and the bonus will help offset your weakness on defense, if you get caught unable to dodge)
If your forces are slanted to the defensive side, your best strategy is to siege enemy provinces (preferably in rough terrain) and let the enemy come to you. They'll wear themselves out throwing wave after wave against your lines, racking up war exhaustion and hopefully ceding province after province to your forces as you win sieges. Wars will be longer but you'll tend to take less battle losses. Most of your losses will come from attrition while sieging. This is why a high Defensive slider is good, because your leaders get increased Siege ratings, which lower the amount of time to complete a siege.
If you've got balanced forces, you can do a mix of both, which is why I prefer balanced forces unless I've got an overwhelming advantage. For instance, in my Hindustani game, I'm well ahead of my Muslim and Asian neighbors, so I'm using Highlander Infantry so I can finish them off that much quicker. Plus, the idea of Indian highlanders (complete with kilt, sabere and pistol) amuses me.
"I am Vijay Chattopadhyay, of the Clan Chattopadhyay...."