Early Campaign

[Gilrandir]

Without a border fort then your own agents get the credit for the enemy agents caught, and valour up quite nicely....

This is good in case you have the spies. At the outset of the campaign you usually don't, so you will face the choice of a lower loyalty + the possibility of enemy's spies and assassins to penetrate your realm (although, to tell the truth, the enemies don't have such harmful agents at the outset either) vs future prospects of multistarred spies of your own. For me the choice is definitely the former - and you can always dismantle the border fort later on to achieve the spies' desired valor level.

But how does that work regarding terrain? Normally I would defend borderprovinces that are favourable for a defence (e.g. bridge, mountains). If I garrison the border only with a token force and keep the real army in the 2nd row, won’t I be the attacker (and thus suffer from my own provinces terrain advantages) if I send the army to take the province back that is being sieged by the invader?

1. If you have a token garrison force the enemy isn't likely to send much more - just enough to outnumber the one he sees in the province. So when you arrive with the bulk of your army later you aren't likely to meet your match numerically.

2. The border terrain the enemy will have to cross invading your province isn't the same as the one your reinforcements will have to cross from your hinterland province into your border province. Moreover, you won't have to take back the province. You will have the message that your besieged garrison sorties out. Of course it will do it to join the relieving force it will see from the high battlements of your stronghold.