When the going gets tough, the tough start digging a place to hide...but the intelligent already have one dug!
Often ridiculed by the enemy for serving no useful purposes, a trench is often the first line of defence for a city. While it cannot fire weapons or shield anything it has to be filled in and cannot be conventionally destroyed, giving the city owner time to mount a defence. Wider trenches not only delay enemies but also stop tanks that lack repulsorlifts and delay armies by creating additional obstacles. Occasionally, trenches are used for the concealment of defenders and can shield them from some attacks. Both quick and cheap to create, trenches are the economical defence for any vulnerable city. These however can be circumvented by troops with jetpacks, starfighters and of course repulsor tanks.