We all know what happens when it's dark outside and we don't have Torches. Give a few minutes and the area will be swarming with Creepers, Skeletons... But what do you do to keep the mood of your Castle, Dark, but not too dark, bright but not in your face? How do you do it?

I've tried a few different things that sound cool, but I'm still working out the problems with it. 

Fewer Torches, with a Nether Brick Castle. 
A "Fireplace" that is fueled somehow... 
Lava, but you make it so that you only see the light, not lava. I have tried this and if I get it right, I'll post something about it.
Redstone Torches
Glowstone
Sea Lamp
Other. 

What have you used, or do you think is a cool idea? Give ideas, I'll try it out, and we'll see how it looks! Discuss lighting of your buildings here!

For this, I use Redstone torches, and every once in a while, glowstone. Though only a couple of times for select places since I normally want it to be bright.

I use normal torches. I don't use red stone well so I don't use those and glow stone doesn't actually light it well.

And I old with dark colors sometimes so I use a ton of torches.

For things like that, I tend to use redstone torches, either in the floor corners or about halfway up the wall, put into niches so that they're slightly recessed.  It keeps things bright enough to keep mobs from spawning, but dark enough to give the spooky atmosphere I want for some things. I'll also occasionally use glowstone blocks in the tops of towers, but I don't particularly like how glowstone looks in most places. It works well if I'm trying to go for a 'queen devil bee' look, though, because the texture pack I use tends to make glowstone blocks look like slightly ominous honeycomb.

Lamps are nice for that eerie effect if you don't want to go with redstone torches.

Depending on how you use them, they can add to the medieval theme of the castle.

Well placed lava can also add to the effect. A specific example would be a moat around a castle. It gives a glow on the bottom of the structure that I like. It's also possible to incorporate lava inside. The only issue with this might be safety, but if you know what you're doing, it won't be that big of an issue.

   To be honest there are many things to use and try out when lighting a dark place and cannot use torches. Though I rarely do not use torches I think I know of a way to get the brightness you want while it still being dark. Like you desire.
 
   That answer is Redstone Lamps. This is because you can turn them on and off whenever you feel like it. Then another plus side is you can have multiple switches to change the mood. Such as one switch turns on a few. Then another turns on some. Then the final one turns on all of them. 
 
   Redstone Torches are also an answer but they seem a bit too dark in my opinion. I would only use those if I wanted for there like to be a light in the distance.
 
   Also I really like your idea for a fireplace. It would be really cool. The only problem I see really is that it might not light the entire room and you do not want to have multiple fireplaces in one room. That would just look ridiculous. Then you could put lights in the room and not separate fireplaces but I do not know how that would go.
 
   My favorite is personally the Redstone Lamps. So try those out if you have not already. I used them once in a dark place and they looked pretty good.

I usually just use torches. if I can't use torches... well I don't play a lot, so I've never really been in that situation. 

Recently, I tend to use torches to light my cave expeditions, placing them frequently enough to prevent mobs from spawning in my wake (I'll usually bring a single stack of torches with me when I leave assuming I have the resources available.  However, when I exit the cave in question, I tend to collect my torches in my wake, walling off tunnels and leaving a single torch where the walled off tunnel is to make it easier to locate again later should I desire to return.

For my houses, the last couple that I have built, I've built a glass tube through the middle, running down through all the floors of my home, then place a lava block at the top so it runs down every story of the house.   Makes a nice bright light source, looks neat, and just requires a little glass to contain/make it safe.

But then, I like my locations brightly lit, not dark and moody, . . .

I use torches when im exploring a cave. But I mostly use glowstone (on creative) to light up my house. Eithers of those items are good for lighting

I'll see if I can get a screenshot of one later, but in castle or dungeon areas, I make an indentation in the wall to put a flame. Looks cooler than it sounds.