1. Construct a path from the start to the finish leaving one square from it, like in the screenshot (you need not construct a straight path like in the screenshot - any way you want) 2. When the fieldrunners seem like they are escaping, pause the game. Remove a tower at the beginning of the path. Close the path at the end where the fieldrunners are escaping with another tower. Like in the screenshot, the fieldrunners are forced to go the other way. 3. If the fieldrunners come close to the beginning of the path where there is a way out, do Step 2 the other way. This didn't work so well as I constructed a straight path. But with longer and twisted paths, you can trap the fieldrunners for as long as you want.