Will the players have the ability to attack and destroy walls/buildings?
In "the olden days" towns/villages etc were walled in to protect the people. Someone was always there to guard and alert the other people of danger.
Protection of the area was thru the guards and the walls - but the walls could be burned or rammed to allow access to the area.
In this game our tribal areas will not always be "manned". Because of players play time, the time zone they are from and the size of the tribe - it will not always be possible to have 24/7 guards. Walls will help keep the inside area safe from invaders.
I'm sure walls will be destructible. If not, then the removal of safe zones & declaring for "war" will be 100% trivial. You'd still have invulnerable safe zones behind your walls.
I could see some compromises like making it so that walls take quite a while to destroy; buildings within walls can't be fully destroyed but only damaged; baskets on the ground can be fully looted, but the attackers can obviously only take what they can carry on their back; there will be a temporary safe status for a tribe after they've been "conquered." While I'm not fully in support of those things, I expect we'll have to make some concessions to keep people from rage quitting if they've lost months & months of work.