Lazybrit is correct, When you log in and you have no gates or towers and you have already built them. You need to log out and log back in again. If that doesn't work then you will need to post it up into the bugs section to be looked at. If they disappear on logout then you will need to post up a bug report. Possibely email support firstname.lastname@example.org
and let them know what is going on so that they can look at your town and see if they can fix it up.