seems something pretty interesting and realistic since there weren't any ways of contacting in the old days. Ah, the primitive people... no satellites and no mobile phone problems. ( there could be an eagle to deliver messages :p)
how about the cancel button for buildings. if for some reason we do not want to build something ( clicked build button due to lag, forced with a gun to the head, superman went through the window and hit the build button etc )
About the troops, i still guess they could still be called the first minute ( reasons mentioned above ) considering that they are walking from the barracks to the city walls and you can yell to them: "COME BACK DUDES, FALSe ALARM, STORMCROW IS MAKING FREE DONUTS"
ps: i hope that the messenger( if implemented ) doesn't leave the tired horses behind :p