I found that turning Wander Spawns (hordes) mode ON is a great way to insanely up the difficulty of the game, to a point where months of survival without hordes are nothing compared to surviving a few days with hordes turned on.
[ul][li]Landmines, turrets, collapsing houses and walls - a deathtrap for monsters without hordes, a deathtrap for you with hordes. The noise these make and the hordes they attract are mental. A landmine can get rid of an annoying critter but cause your base to be overrun in less than an in-game hour, forcing you to leave resources behind and run for your life. This proves to be a challenge early on, as every resource is important and carrying all of it on yourself is usually not an option.[/li]
[li]Setting up a permanent base - any slight noise gets your base overrun and as such building a permanent base is very difficult. Even if you do manage to find a quiet place to set up base at, hordes wander from place to place and the occasional zombie, with the way hordes work at the moment, can create a horde of its own and fuck you over.[/li]
[li]Early-game challenge - a horde can stand between you and the way to success, whether it’s a sporting goods store, a gun store, a lab or any other desired building. Even if you’re on the move and the hordes aren’t a danger to your own place, they do pose a huge threat to anybody trying to pass through them. That leaves you with 2 options: either leave the place alone (or come back later when the horde is gone, unless the noise it creates keeps it there) or go through it, endangering yourself and creating more noise that will attract more zombies to the area. There is also the certain scarcity in resources mentioned earlier, with hordes forcing you out of your loot stashes and out of building with valuable items in them.[/li][/ul]
EDIT: I forgot to mention that you should turn Experimental Z-Levels ON as well. This ensures that holing in underground doesn’t solve all your problems; with Z-Levels on, time isn’t frozen on different map layers and as such zombies can attract other zombies while staying underground and creating noise - basically a time-bomb waiting to explode into a horde in a form of constant 'whump’s and 'clang’s. Monsters also follow you downstairs instead of staying frozen waiting until you emerge again, prepared to deal with whatever they throw at you.