It doesn't offer a ton of protection, but it's often enough to keep enemy villagers from making use of it themselves, since the tower will kill them off or destroy any structures they try to build before they can finish. I've also found that even if you're not ready or capable of exploiting a resource at the current time (such as a gold mine or an empty settlement), try to build a guard tower next to it if you can.
All food sources (except for hunting) are now infinite, so once you pick a fishing spot it's yours and it'll never run out of food.
If you can do it, fish like crazy to get food.
