Isn’t it possible to creat your own defence?
Yes. Each Sunday a new tourney is announced. You have 24 hours to set your defense for that tourney and then it is locked for the week - no more edits once the tournament starts.
Whenever a tourney is announced, usually on Sunday, your are asked to set up/create your defense.
With start of the tourney on Monday it is fixed for as long as the tourney takes, Saturday.
The same with your war defense. When war starts it is locked until end of war.