Clash-of-clans – Clash of clans war during preparation day

The troops in the clan castle indeed defend each attack.

You can be attacked more than two times in a war, but your clan castle is always full for defending against those attacks, full of whatever was in the castle at the end of the war's preparation phase.

Additionally in war:

1. Heroes are always active to defend your War village.
2. Defenses like Xbows and Inferno towers, and traps are preloaded
3. Any defenses under construction are active in war, unlike your home village. In fact, any defense STARTED prior to the end of preparation day is active and defends your war village.

EXP in Clash of Clans is pretty much like experience in any other game.
It can be found on the top left-hand corner of the main screen and profile screen.

The experience you gain then is placed alongside a level system, with EXP cut-offs, a number that increases as you increase your EXP, but the requirements to get to the next level also increases.

It basically shows how much a player has invested into a game. Or for most MMORPGs; their character.

You pretty much earn experience by:

• Upgrading Buildings
• Donating Troops
• Earning Achievements.
• Destroying townhalls in attacks
• Clearing plants/rocks from your village.

For all we know, there could be a Level 30 player with a Townhall Level 8 in Challenger League amongst all the Level 90+ players.

Answering your question:

It basically shows how much a player have invested into a game.

The only thing EXP is useful for is for unlocking decorations.
The more EXP you have, thus giving you a higher level, the more decorations you unlock.

If you need more information, you can find them in the Wiki:
http://clashofclans.wikia.com/wiki/Experience
http://clashofclans.wikia.com/wiki/Decorations