Level 13 walls + the Kremlin (20%) + a Wall Blessing (30%) + Sultan Saladin Wall & Gate Hitpoint research (30%) equal walls impenetrable by a basic Elite Combat Engineer. Protect that silo and those resources with THE WALLS OF TROY :-)
All defensive buildings have a range indicator when clicked after placement. I suggest tactics indicate their placement point with a bullseye and affected range with a circle on the initial click of the battlefield and are then activated upon a successive click of the bullseye. Not clicking the...