I've already taken down the mountain I'm on by about 10 blocks (which equaled 15k dirt and 5k stone) so it should be enough for a decent looking castle. I decided to use the brick stone because I figured it would look the best for a castle on top of a mountain.

Can't the PC version have friends join in your game?

And yes I would like to see an underwater rail tunnel lol.

Aye, I've done obsidian casting before. It's pretty easy, except for that I have very little lava around me right now. The bits of lava that are coming down the walls in my Alcove Castle are about the only lava around. A tiny lake of only six or seven blocks that I emptied. I'll still go hunt for them, but still, pretty annoying. heh

Good call on knocking the mountain down some. Should be enough room then, I'd hope.

And yes, you can have others join your game on PC, but it's limited to those on your LAN. In order to get someone to be able to look at your world otherwise, you basically need to set up a server on your comp, open the port to the internet using your router and host it, unless you want to pay for a server at one of the hosting services. Short distance is easy, but long distance is a pain.

And here is the underwater rail tunnel. Was pretty fun to make, actually. I created it because I really hated having to swim over that lake, as it was just large enough to be annoying. Anyways, here it is: http://imgur.com/cKwFCCP

It does seem that whenever you need lava you can never find it (and every other resource in Minecraft) and the only time you see it is when you don't need it.

That's a pretty cool underwater railway BTW. Working in the water is such a pain but it's much better than swimming accrosse every time you want to go somewhere.

Used to be that it was only you in single player games. Multiplayer games would be on servers only, which would be separate from single player unless you specifically moved your world file over to the multiplayer server directory. It's much better now, at least.

I did actually manage to find some lava now, so, my castle is about done, I believe. Time to move onto other projects! Not sure what yet, but something...

And thanks! Took a bit to do, but worked out pretty great! I don't use it a lot now, due to my Nether rail system, but it was still a really early build for me. Was pretty fun and worked great. Maybe I should consider an underwater city or something for my next project. Hmm...

YES! Do it! 

Is there a way you can make yourself not drown underwater?

Outside of using blocks in certian patterns to make safe areas where water goes around and the use of Resperation enchantment there is no true way to avoid drowning.

YES! Do it! 

Is there a way you can make yourself not drown underwater?

I might have to build it, it does sound like a fun project. heh

Not really on the drowning thing. Blacksaber mentioned the one way to at least increase the time of drowning, but I tend to do it in a fairly simple way. First, I build a small structure right in the middle of the area I want to build on. 4x4x4 typically works. Make it solid. Then, while going up to get air and then sinking down, you knock out a few of the blocks out of the side, just enough to make an air hole inside the 4x4x4 cube (or, better, knock out two blocks and place a door on it, which creates an air pocket). Once you do that, you hollow the place out and, voila, you have an area underwater that you can survive and build from. So long as you don't destroy the roof, there'll always be at least an area to work from. You do the same in other places, basically creating a series of areas that you can go between for breaths of air. Then you prep the area and start building. Works pretty well!

Do you have to make an underwater structure solid and then mine through it to get the water out or can you simply enclose a section of water and the water will go away?

Nope, has to be completely solid. See, every block of water in an in-game ocean is an individual block. So if you just make a structure that's hollow inside, it'll still have all of the water in it. If you make it solid, then mine it out, those blocks you placed erase the water blocks and the structure will be empty of water. So, you make a solid block, mine out the side and the inside and you have an underwater breathing area! Pretty easy to do it.