Raised walls are good. Any walls/barriers will slow mobs down so you can get out there and deal with them if needed. But for tough mobs/larger raids, if you don't want to have to run out and handle it, then I agree a trench around your base is the most useful along with some kind of wall (raised earth or otherwise) to break line of sight for ranged mobs.

Eventually there can be flying mobs (mostly bats or drakes) but those are generally pretty easy to deal with.