What if the optics get damaged? What lasing medium are you using and what if it leaks?
For long-term defense systems, I'd try to go as low-tech as possible. Every bit of technology of the past 300 years is tied to a rather large manufacturing & supply chain. If your turret has μC in it, where will you get a new one if the marauders happen to score a lucky shot?
You can reshape the question into how small could you make a microcontroller fabricator which can produce all of the electronics within itself? One square mile to one square foot - it seems likely that the best possible is within that range. How well could it be optimized?
