I get the idea though - maybe it could be something researched or upgraded, like the beacon tower in Evony, still, I though the beacon Tower thing was much too obvious. Possibly, you could get warning of an incoming attack, but no info on it. Perhaps you could have the chance to send "scouts" or something to figure out the incoming army. That sounds more realistic.
"Yes... I am writing a book. So far I have the page numbers." - unknown author.