The red arrow at the lower right points to my evac zone. My templars are on the left. See the turret I dropped near the center? It is totally preventing my templars from reaching my evac zone. This is a maze level, where I need to search for 3 artifacts. There is no other path to the exit. I placed the turret when all the objectives were to the left, thinking I was very clever since it would block any enemies on the right from reaching me. After finding the artifacts, the evac zone was marked; I did not know in advance where it was going to be.
I had to twiddle my thumbs for 4-5 turns while the low level skitters next to the turret whittled it down and destroyed it. Only one can reach at a time. I tried shooting the lowest level skitters so that something higher level could reach it, but that didn't seem to help. Some turns, nobody moved up next to the turret to attack at all.