Build about fifteen and twenty lasers near each other, then select one of the lasers near the middle and choose the "Link Nearby Lasers" option.
Linked defense towers hint: When enemies approach, link plenty of defense tower together. You can link them by pressing X or by dragging one building to another. The building that links to the main building can also get other buildings to link to it, thus giving more power to the main building. The linked building has excellent range. Note: Unlink a building when an enemy force gets very close so that it can defend itself.