Nature invented wings so it didn't have to care about moats.

It really depends on location of your base, your phase of progression and your personal preferences.

Easiest way to deal with any currently available invasion is to pop the Biomass power (swamp boss ability) and jump into the fray with an effective loadout. Some exceptions to that exist, in case the invaders wield elemental attacks, and that's where ate-game materials come in. Stone is much more resistant to those, and marble (late-game material) is outright immune to all elemental damage and resistant to all physical damage except pickaxe (If you look at the concept art for ashlands creatures, you can see there's a skeleton with what looks like a magic staff. What kind of magic do you think they wield in the fiery biome?). You can check the Fandom wiki for HP values and resistances for various structures, it's the only feasible way of discovering how different materials behave.

The likes of greydwarves or draugr, especially bowmen, can be kept away by stakewalls, multiple layers if need be. They have as high HP as stone wall pieces, but are considerably cheaper and easier to construct. For all other major threats from there on, you will likely want stone...

At this point, if you can build a proper moat, it should help, but as said above, winged creatures exist.

Another option is to fortify walls of any kind with iron cages. It's expensive, but saves space and helps a lot against anything which is affected by cleave penalty as well as any non-AoE projectile attacks.

Finally, you'll have to consider rain degrading integrity of even iron components. Rain won't touch stakewalls, stone or tarred wood, so you can do something like this:

https://steamcommunity.com/sharedfiles/filedetails/?id=2935749071

or this:

https://steamcommunity.com/sharedfiles/filedetails/?id=2935749150

These are just examples of how rain affects pieces, not a comprehensive defensive measure for any threat. It's hard to see if iron cages are damaged with the poor lighting, but hopefully you get the idea about protecting your defenses from rain. You could also use roofs of course, but I find especially the former type of stacking of multiple layers better. It takes less space, but won't protect from AoE splash attacks as well.

So the most comprehensive option is to build a variety of defenses and spacing them out for defense in depth, all the while not forgetting active player participation (despite what Hugin says about defending). You won't be able to do that right away, but it's something to consider for late-game progression.