1. Forts are DC, Keeps are EP, and Castles are AD
2. If you are a Templar (I don't know about other classes), healing buffs also help when repairing walls. Throw down purifying ritual and then fix the wall. You'll get more bang for your buck.
3. You CAN heal a wall that has recently taken damage if it is within the first hit or two. If the wall has >~98% health, you can fix it even if it has taken damage. In order to prevent the permanent wall fixing (we've been hitting it forever and it's still at full health!) you will need about four trebuchet shots to hit at the same time. Coordinate! Once it's below that 98% threshold it cannot be repaired and you can damage it as normal.
4. You can and will fall through the grate on the second floor of the outposts if someone hits you with certain skills. Be careful.
5. When looking at the map you can push X (on ps4) to get the resource/keep stats. If the resource is level 0 it has changed hands recently and there are likely opponents nearby. Resources upgrade every 9-10(?) minutes to a max of level 5. If the farm reads six minutes until the next level, and the lumber mill reads 9 minutes until the next level you know they hit the farm first, then lumber, and are likely on their way to the mine.
6. If you run into a resource tower player horde, DON'T take the flag! Use siege to knock down the tower. If you can't do that, stop running in there unless you have overwhelming numbers. You will get killed, a lot.
I've been playing this game for a while now and some of these were news to me not too long ago. I hope this is helpful to some.
If I've missed something or made a mistake, please let me know and I am happy to change/update this.