The 'wall of fortified cities' is nothing more than to add some income to my nation. The eastern part of the world isn't rich on farmlands and we need such improvments on the few we have.
We don't see the whole world, but we've reports that T'ien Ch'i also be big and mighty. Look at the turn files of the last 10 turns. T'ien Ch'i was very often the king of turn file size
.
(And we were also in war with vanheim and had many battles so battles alone can't explain such a big file
).