OK, let say your tribe got 5 members and mine got 50. I will be able to kill your guards really quick, and when you comeback, you wouldnt find a thing
Its a nice idea, but needs lots of thinking on that

My suggestion would be, implement a defense mechanism for online/offline tribe members.
Let say a tribe have 1 member.
Offline defense system is 10,000% for all objects on tribe territory.

Let say a tribe have 2 members now.
Offline defense system is 10,000% - 150% for each member offline.

So, if tribe got total 10 members, and 5 are offline, then
the offline defense system is 10,000% - (10*150% = 1,500%) = 8,500%

If a tribe member is online, then the offline defense system is
10,000% - 300% for each member online.

Thats would create a balance between players online and offline for tribe territory.

Let say if a single brick wall got a 2,100 hit points, then its offline defense for a one person tribe is 2,100 * 10,000% = 21,100 hit points.

For a one person with a good weapon you can inflict around 300 hit points per minute.
So how long will one person (A) destroy that wall when the character (B) is offline??