Can't attack city

Hi.

in my last game i couldn't attack a city. although there was no wall around it or something similar. i even stand with my unit above the city. i tried many different ways to attack the city. but no chance. here is a pic:

Was the yellow player your enemy? Did you try attacking it from the top?

This often happens to me. I just refresh the page and it sets things a bit off to a point allowing me to go into cities like you tried to.

Yes yellow has been my enemy.

and i tryed it also from the top, but no didn't worked either