How can I improve my Defense

[Hieronymous Alloy]

Darkness bonus is no longer a thing as of A16.

[DeathWeasel]

That's good to know, thanks. I'll need to redesign my defense.

[Thyme]

Walls provide better cover than sandbags. Sappers need to be dealt with with the old way, you can simply intercept them. They walk through the hole one by one (more or less), usually giving you enough time to kill them as they diffuse into you base. Just place your pawns in range of the mined entrance.

As for the darkness bonues, I'll continue to use that, as it still helps with some bad weather penalties, no? And I don't like my colonists getting soaked up, I consider it a mood buff for myself. Oh, and in A15, when the surroundings of an enemy is on fire (gotta love incendiary launcher), the darkness bonus does not go away.

[Listy]

Wouldn't sandbags provide cover for the raider?

Yes, but Sandbags provide negligible cover. Compared to you hunkered down behind a 1m thick wall with oodles of cover. Plus if there's cover, no matter how slim it means the enemy don't rush you.

[Catastrophy]

Usually I go with some engineered form of entrance. I'd place a rock chunk on edges where I don't want them to take cover. Once a trap spings the next guy can just safely stand there.
I also sprinkle my base with single pillars to fall back to and have cover should the entrance get overrun. A tiered system of walls and entrances also works and buys more time in case of falling back.
The OP base is too open. I always look for a decent set of natural cover (mountains/hills).

I wish we could use water better as defense. There is always some shallow part that allows enemies to traverse.

Also, the entrance is too wide, you don't want to have them come at you in a wide front. The Spartans held Thermopylae, which is (was) a natural chokepoint.

[LordMunchkin]

You can exploit the pathfinding of enemies by lining your walls will traps. It won't work against grenadiers but it will massacre melee pirates and tribal guys. One way you get around grenadiers destroying your defenses if by having different doors into your base. Raiders will usually split up if they can attack multiple doors. They also love running right next to your walls when they're going to the next door so that's also where you should put traps. Compartmentalization and the good old IED in every room is nice too (turrets cans substitute for IEDs). Finally, having a colonist, someone fast and with a long range weapon flank them is always FUN*. Hit one of the attackers and some of them will split off to chase your dude.

More defensive tips include using trigger happy minigun guys in tight spaces, brawlers to intercept enemy melee dudes, and careful shooters to flank and snipe grenadiers/rocketeers. Incapable of fighting guys also have a purpose. Put a shield and armor on them and send them into cover at the front. It's not necessarily a sacrifice as with a little micro you can have a bunch of dudes chasing your pacifist like a chicken out of the coop while your shooters nail them.

*maybe not a good idea for the faint of heart

[Hieronymous Alloy]

Wouldn't sandbags provide cover for the raider?

Yes, but Sandbags provide negligible cover. Compared to you hunkered down behind a 1m thick wall with oodles of cover. Plus if there's cover, no matter how slim it means the enemy don't rush you.

This isn't quite true: sandbags provide the best cover you can walk over (although walls are better, as you say.). The real answer though is that you can negate that cover with flanking fire from the side.

[Catastrophy]

...

More defensive tips include using trigger happy minigun guys in tight spaces...

OMG! Yes. I saw it all in my last colony - but not ze trigger happy thing. Now it makes sense!