PC "Hard mode", eh? Then where did all the baddies go?

I just finished loading my fort up with a myriad of traps and miles of wiring connecting dozens of pressure plates and switches. I also just defeated the Wall of Flesh and unleashed "the forces of dark and light".

Now my fort sees no more enemies. No stray zombies, no flying eyeballs (it was tricky building a trap effective against them, but I did it.) I haven't even had a Goblin army show up in ages.

It's boring now, and disappointing. Where did all the enemies go? Will there ever be more?

The more villagers you have in your "town," the less monsters you will see. If you want to see some monsters just venture out a bit from your base, they might be getting killed by the traps. Btw, you should have noticed this earlier in the game, even before hardmode.

Well then wait until the blood moon rises. Then you'll get your baddies.

Move about five or six screens away from yourNPC-congested lair. There you go, a whole horde of weak-ass, overly predictable and squishy bugs for you to crush to your heart's content.

So the solution to my problem is to abandon my fort and start another somewhere else? That sucks.

I spent hours carefully laying out my fort. Every entrance is carefully shaped, every one of the 25 dart traps is painstakingly positioned and wired up. But now, none of that will be used. It's pointless to arm your fort and useless to build defenses.

Defending from attackers is fun. This isn't fun.

Then were you planning to spend the rest of your days sitting in a fort all day? Oh, I'm sorry. A fortified fort.

This is not the Tower Defense game you're looking for... //Jedi Mind Tricks.
Not the first time someone complains because they did it right. Doesn't make it any less amusing for me, though. Also, even at its highest points, Terraria's native spawn rate is low as crap. Almost like Oblivion's "one wolf a mile" spawns... Good thing there's mods to solve that flaw... In Oblivion.

Either you move, or the NPC's move.

Also with some experience i could say that thin layer lava traps are way better than dart traps.

Defending a castle is fun gameplay. Period. Anyone who says otherwise is being an annoying contrarian.

Especially early on, a BIG part of this game's play mechanics involve building walls, ceilings, secret tunnels and other implements to protect yourself from all of the dangers outdoors. This is not some crazy left-field suggestion I'm making here; This is already a core gameplay mechanic. Can you not see that?

My point is: this gameplay mechanic isn't perfectly designed. It's marred by this disappointing flaw.

Of course not! I never implied that. Yes, I visit my base often to organize, offload all the junk I collect, and to build and shape my home. And while I do these things, it's really rewarding to see my carefully constructed fort actively keeping me safe. But like all of you, I still spend most of my time out exploring.

Yes, the beginner monsters are easily defeated. But it was my understanding they would get more challenging as time went on, not disappear completely.

Yeah, you make a lot of sense. I suppose I can build a forward base somewhere more dangerous. But then the big question is this: how much of my base am I able to bring with me to the new site without affecting the enemy spawn rates around the new area?

I still use the Guide a lot, so he'd have to come along. And I need to be able to sell my loot, so I'd need to bring at least one other NPC. Can I bring two NPCs? Will that hurt the enemy spawn rates?

Yes, NPC's significantly reduce the spawn rate. So ideally, stay in your uber base and kick them out ;-)

..oh but 2 might be ok.

If you have a King statue, you can wire it to a switch and use it to summon a random male NPC to your location (useful for selling loot). Then when you move away from them, they warp back home. Same for a Queen statue/female NPC.

Your attackers come only at night, and thats also pretty minimal
If you want some action, make the surrounding hollowed, that way your NPCS will stay there and you'll get...
Pixies to defend the base from?
At night the gastropods seem fun to deal with, with their lasers and everything, but as long as your NPCs are around enemies will stay away from the base, they come in smaller numbers.

Don't be a wimp, run away from safety and smash some altars. Then diggy diggy hole and miney Killy miney die!

I made bases in most biomes, yet I seldom visit them. I have a vault world for crafting and storage. I think I've spent eight hours (non-stop) mining the living hell out of hell. But don't be fooled, the only ways for hardmode mobs to be challenging are that the player is unprepared, or unskilled. They may or may not be mutually inclusive. The worst you can get is the broken armor debuff, but it's easily countered by avoiding hits for five minutes.

If your house was never safe, it wouldn't be fun for people who DON'T play the game as a fortress survival game.

NPCs slow/prevent monster spawns, so move them.