This happens when 2 conditions are met on the War tile:
- There’s no enemy players in that tile, but just the one assigned as Tower Guard.
- The enemy player assigned as Tower Guard was kicked from (or left) the alliance.
Since the player is no longer in the alliance and there’s nobody else to attack…
You can’t attack anyone.
Yup, player has left the alliance more or less on voluntary.
This sounds like an interesting tactics. If you want to secure a tower the Tower Guard has to leave the alliance and nobody is allowed to walk on that field. As you cannot attack, the war will end with a 0:0 and by default the defender is winning in this case.
I am still checking with the devs about this. I wll get back to you.