This game permits PvP and attacks are a part of the game.

People with power seldom want to lose their power in attacks on their equals.
They are attacking you BECAUSE you are weaker and keep attacking you because they get stuff from you
(or because you piss them off with your reaction to them).

So react reasonably, and set your city up to keep them from getting anything from it.
See the section titled “DEFENDING YOUR CITY.”