Here's my quick take on your questions:
Understand I've played quite a few PBEM games but the people I play against are 1) friends I know personally, 2) Not as aggressive as I am (which is not very) and 3) Not very good.
I've never lost to them and I'm not nearly as good as many of the people playing this game.
So understand that these comments come from a very warped perspective of PBEM.
Hopefully they are still helpful.
#1. When do people on multiplayer generally start doing military build-ups?
My experience with humans is the big hit either comes right away or with the advent of a high-movement/high-attack unit like Knights or Cavalry.
#2. Does almost everyone do the settler Spwral
Yes. If you don't you lose horribly.
#3. Does anyone know anything about the enemy tactics or common tactics?
Most common war tactic is the SOD (Stack of Doom) if you don't send things in groups humans will eat you for lunch.
It's also very common to try the "rouge state" strategy turning every player against the person you're at war with. This works better with the AI in a game but can also be done with humans.
Conversely it's also common for humans to wait and see who is losing then try to aid them
make it more bloody and costly for the winning team.
That is
until the person is obviously beyond hope
then the blood is in the water, and everyone scrambles to get what they can from the poor sap that's going down.