1. Do not, for the love of Talos, put in a gate, or portcullis or something, because you really don't want to sit safely behind your walls when you can have your enemy storming inside instead. Build a couple of barricades instead, that will hold them.
2. Make sure to spread all your archers out thinly, and put them on exposed positions along the walls, don't let them take advantage of cover when defending a fortification, and don't let them release barrages of arrows, that could stop the enemy from advancing.
3. So those cunning barricades didn't hold eh? Well now that they are inside, do not make the mistake of trying to defend chokepoints and take advantage of higher ground. Make sure your defenders are spread out instead and allow the enemy free access to all parts of your fort. Everyone knows that preparation and organization are the first steps to failure.
4. Don't use any form of magic, like runes, or other technique to assist your defense. Like that oil that you can find all over every barrow and dungeon in Skyrim that burns so nicely, surely you can't come up with some good use for that when defending a fortress.
5. The ancient nords were obviously very cunning in designing different traps and contraptions to keep nosy people out of their sacred resting places. Modern nords and imperials however, find such innovation and engineering quite overrated when defending your own life.
Please make sure to spread these important directions to every commander in the region, our success in this war rests upon the proper adaption of these strategies.

