Moats don't have to be water filled... They just have to be pits capable of stopping/collecting an attacking enemy so they're easier to kill. I'd not let zombies live in my moat, I'd be setting them on fire asap.
Or using a spike on the end of a pole to just poke 'em in the head one at a time.
If the pit's big enough getting out would be almost impossible for even a competent human much less a zombie. They seem to not really go for climbing chain link fencing either... If you can put a perimeter of that around the house, more the better.
The problem with the moat/trench is it'll require maintenance. Trenches collapse if you don't shore them up, erosion and things would hurt them over time, but it just means you maintain it ultimately.
Wouldn't hurt putting some zombie debilitating boobie traps around too... Simple traps that incapacitate them, or at least limit their mobility (thigns that go for the legs in particular).
Also though, back to the moat with water, actually teh zombies do need to breathe for some reason. The one at the CDC showed that its respiratory system kicked back in... it just seems to work despite massive trauma so I don't know if water would impact them or not. They're not afraid of water, at least to a degree (flashing back to them attacking Darryl), but I know in Romero's zombie world they supposedly were afraid of water at one point then decided to walk right through it... I dunno. But water would actually be more of a problem than something I wanted in my moat. I'd want my moat more as a collection zone for just an easy way to kill them efficiently and with relative safety.
Walls are great too. Fortifications... but a collection point solves multiple problems. It's defensive protection for the house plus it's also a trap for ease/safety in dealing with the problem. Plus the house could be your ultimate fortification, assuming you get that lazy assed group of people in gear and actually fortify its lowest levels. Add in (assuming you have an unmolested Lowes nearby) a chainlink fence on the house side of the moat... Wow. You'd have some pretty solid safety.
If you have the means and people, and outer wall around the moat would be even better/more helpful.
Fortifications throughout history have ALWAYS been about layering, high ground/sight advantage, and efficiency in defense/killing.
It's funny how the zombies really would make you want to take a medeival approach to just about every aspect of offense/defense... The living are the ones that would make you have to rethink that though. Especially if any of them have accumulated military hardware.