[Dedas]

I understand you.

You ask how much infantry you need. I say a lot but not as much as you think assigned to a defensive role. Thanks to castle defence bonus of Ghoul Guardians and Halberdiers you effectively halve (and more) that number. But it will cost you some upkeep, most of which is covered by the fortress administration bonus on income of course, but that is thanks to order.

Another question is do you need defence in your castles? I would say yes, and that is because you've built a lot of them. Else you'll be giving your intelligent opponent a safe way into your kingdom.

You think that you will mostly be needing defence in your frontier fortresses but in a game like with stealth and teleportation I would rather be safe than sorry. 20 Halberdiers (that will count as 40 with strength 11) in each fortress is not a huge cost and it will buy much needed time for you troops to reach them. Or why not just tie your enemy up while you attack their land? In the meantime you can harass the besieging enemy with nasty blood magic, Vampires or other summons.

But to answer your last question. Is order 3 needed? Well, no. But it will speed everything up for LA Ulm. That is because this dog does not do one trick only, you need a bit of everything to beat a strong opponent. Fortresses, troops, combat mages, blood magic economy, patrollers the list goes on. If you take turmoil and luck you might or might not afford what you need, that makes it hard to plan ahead. LA Ulm is all about planning ahead like almost every slow moving nation. You could try to ignore this and try to play it more extremely by focusing on one of the faster aspects like rangers but that will make you very vulnerable if the enemy has good intel (like one should).
You want to reach the point where castles cost nothing and summons rule the earth, but to do that and be best at it you need a big and stable ground to stand on. Some nations get this by speedy scream attack troops and by a nimble defence that allows them to have more room between fewer fortresses. Turmoil/Luck goes will with them as when they have gold they can ship their new troops fast to the front. Ulm however... well they need time to get to that point, and any setback they will feel double, so to make that time shorter and safer they need a lot of steady gold.