pilou said:

I have been attacked on a tile, by an invisible attacker...

Is this a bonus ? a bug ? a cheat ?

If it is a bonus, how is it available ?

 it is server lag...

from time to time refresh your browser, will solve the problem

server lag lead to other problem as tile show being empty but when you wish to send troop, it say tile don't exist anymore or is already busy

There is a other thing possible if the attacker was in the forest... he have launch the attack but was hit by somebody else after attack was launched... this lead to a random teleport for his city if he loose the defense but march launched will continue to the target... and then return to the new location of the attacking city