All they needed is a moat

But they are idiots.

They've been living in the same places for god knows how long and the only defenses they've managed to build are walls.

Actually they didn't even build the walls. They just inherited places with walls.

Now we apparently come to a showdown with Alpha, where their plan appears to be fighting them in an open field outside of the walls where they can easily be surrounded.

They tried that in some 70's disaster film about a pague of ants marching across the country .
It didnt work then!
even with tiny little ants!

Boat? Boat with explosives? Drones with explosives? Helicopters?

A moat would solve the day to day zombies. A horde would fill up a moat in no time. Zombies would layer on top of each other and fill it in forming a bridge. Then you're stuck.

If you ever played 7 Days to Die, you'd understand why that won't work. A few examples.

One is mentioned above.

Scenario #1: Walkers fall into the moat, get up and start clawing at the walls until the ground above is dug out then causes a cave-in.

Scenario #2: Walkers fall into the moat (with or without water) and start piling up until they build a bridge of bodies to walk across.

One of the reasons moats worked back in the day was the fear it gave the attacking enemy, they didn't want to fall in and drown or die from a fall. But Walkers don't have this fear.

Are moats easy to make? They didn't have many engineers around. They had to learn about windmills, plows, etc. from the lady who wanted records.

Assuming you could dig down ten meters without hitting bedrock - and in most places you can't - those far as the eye can see megahordes could fill it with squirming bodies. Then there's the matter of keeping it full of water. Not so easy to do without modern municipal water supplies. Alexandria has a reservoir, but would it be enough to keep a big moat continuously filled? Might be difficult in the summer months when water levels get low. Plus you'd need to make sure the water flowed only one way, from the reservoir into the moat, if they mix freely you're going to get putrid rotting flesh contaminating your drinking supply with bacteria.

What would happen if your moat absorbed an entire herd and protected you? You've now got a whole bunch of "live" zombies that aren't so easy to get rid of. How do you clean them out? Let the moat dry completely and then burn them? Leave them there as an obstacle? A pit full of walkers that can't be lured away would be tough for any attacking people to get past. Of course it would complicate your own comings and goings.

The best defense against the dead is a very high wall. You could pile dirt (or gravel) up to about half the height of the wall on the inside of your community. Earthworks like that would make it impossible for any number of walkers to push the wall down, and even people crashing cars into it wouldn't do any significant damage.