I'm running a ruined tower/castle thing filled with undead and a necromancer boss this weekend, and I could do with some tips on how to make it as creepy and scary as possible. Like, descriptions, atmosphere, etc. Thanks in advance!

Lean into all five senses in your description, especially smells and sounds. Cobwebs will get stuck in their hair. Insects and rats will crawl over their feet. Ceilings, floors and walls can be unstable and crumble with a touch. Mildew is so thick you can taste it in your mouth. The dampness sinks into their bones until nothing they do can keep them warm. Lower the lighting or use candles for added effect. If you're in person, Tabletop Audio is free and fantastic. It has a number of soundtracks that fit a dungeon theme. It also has soundboards you can utilize.

Sandy Petersen, who wrote the original Call of Cthulhu and some of its more famous campaign adventures, explained how to do this at Gen Con a few years back -- there is a video of it. As Buzzard says, he advised engaging the senses, and particularly not just sight. He also pointed out that in addition to the "regular 5" senses, sense of heat/cold, pain, etc., are also there. He advises using 3 senses. "The air suddenly gets cold.... you feel a sense of pressure from above... and a foul odor assails you, the smell of rotting bodies." Etc.

You can search his video and watch it for more -- it's like 1.5 hours long. He asked for random things to go with it and some young lady suggested a "giant bunny rabbit" as the "entity." He actually managed to make giant bunnies quite terrifying. Off the top of his head.

The guy is gold, when you want horror or scary stuff.

There are two tips I always give DMs who are new to horror, and they’re not what you’d expect.

#1: Get your players to buy in. If they don’t know they’re supposed to be scared, or don’t want to be scared, they won’t be scared. But run the idea of a creepy adventure past them ahead of time, and make sure everyone’s ready to get creeped out, and their brains do half the work for you. And if you really plan to scare, plan some hangout time before the session to get the laughs out, and after to cool down. Trust me on that: otherwise players can laugh through the game.

#2: Atmosphere is everything. Everything. How you tell a creepy story is far more important than what it’s about. Dim the lights or swap them for candles or red bulbs. Turn down the thermostat, and swap the couches for hard chairs: mild physical discomfort is a powerful psychological tool. Turn on creepy music, and speak in a low, quiet monotone, like a creepypasta or true crime podcast, so your players have to be quiet and limit crosstalk in order to hear. And don’t try to be loud or do jumpscares: just focus on creeping dread.

Now, the extent to which you utilize these tips depends on whether you just want a creepy feel or want to go full-on horror. Either way, though, I hope they help!

Have some creepy stuff going on that the party can't solve by hitting it hard or throwing fireballs at it. Like deadly creatures they have to creep around instead of fight, or ghostly appriations etc. Or something they have to run from and can't fight directly. 

It takes a lil bit of effort, but preparing a list of sounds that fit certain events and encounters can help... So if the group meets a snarling dark monster, have a sound clip of that's similar to that and play it (preferably in a subtle manner so that the players don't see you reaching for play the button)..

That and some strong ambience sounds can do a lot.