"Moat" actually means "ditch." It's actually better to do it away from water so you can go deeper. Both edges need to be steep/deep enough that you can't easily jump out of it. The outer edge needs to be several square widths away (maybe 4 or 5? not sure of the exact length) from your structures and gardens to avoid a troll's AOE ground slam. That will stop them from walking in. They may throw rocks if things are too close to the edge. It won't do anything for enemies that fly.

Leave a small gap in your bridge so they don't come in that way. If the gap is small enough you can still walk across while also breaking their pathing.