moat

1. A deep, wide defensive ditch, normally filled with water, surrounding a fortified habitation.
2. An aspect of a business which makes it more "defensible" from competitors, either because of the nature of its products, services, franchise or other reason.
3. A circular lowland between a resurgent dome and the walls of the caldera surrounding it.
4. A hill or mound.

verb

To surround with a moat.