Ok. You will spawn in a castle/ruin/fortress thing, made of stone and cobblestone and maybe some wood. Your objective is to stay in this castle, add on to it, rebuild it, (more on this later) and try not to be killed by all those mobs you love, like ghasts, and even lightning creepers.
What's the catch? Sounds easy.
After 3 Minecraft days, Monsters will spawn a lot, even during the day. It gets a lot harder, trust me.
Still sounds easy...
Well, monsters will have the ability to break through blocks, except for stone. Skeletons' arrows, when they hit stone, after five hits, it will become cobblestone. 5 more hits, it's gone. As time goes on, you will find this frustrating and time consuming to rebuild it.
You will be mining obviously because this is Minecraft. While mining, you will find all the things you need, such as stone and ores. But while mining you may come across little chambers with chests in them, containing supplies such as weapons, food, and many other useful items. Trees and mobs such as pigs and cows and chickens and sheep will spawn as usual, so that clears up the food question.
While defending yourself, the spawning schedule goes as follows.
Day 1 - build
Day 2 - build
Day 3 - finish up your base
Day 4 - zombies
Day 5 - zombies and skeletons
Day 6 - zombies, skeletons, and spiders
day 7 - Zombies, skeletons, spiders
Day 8 - Zombies, Skeletons, Spiders, Creepers
Day 9 - Zombies, skeletons, spiders, creepers, and a ratio of 1 lightning creeper to 8 regular creepers
day 10 - Zombies, Skeletons, Spiders, Creepers, Lightning creepers, Ghasts, Endermen.
This will go on and on, until when you die, when it is game over for you, and your score is determined from the number of days you survived (not counting the 3 first ones) and the number of mobs killed.