The Demoman may be a great offensive weapon, but as far as defense goes, there is nothing better than the Demoman. On defense, you must always be vigilant, but as a Demoman, even more so.
I mentioned in the offense section the idea of hiding your pipebombs. Well, that's basically what you do on defense. A cleverly hidden pipebomb trap can protect your flag much better than a Soldier or a Sentry gun can.
Hiding a Trap
In the basement of a map such as 2fort5, there are numerous places where you can hide a good trap. Just strewing the pipes about the entire basement is dumb, because an enemy can see them. I usually lay two traps, each with three pipebombs in it. I lay them around corners or by the sides of a doorway where no one would see them until it was too late. Here is an example in the basement of 2fort5:
click on the pictures for a larger image
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| Put traps in clever spots... |
...where people won't see 'em 'till it's too late |
Of course, different situations require different strategies. In some cases, it is best to make it apparent to an enemy that there is a trap, in order to delay them - such as when your flag has been moved from the flag room and is located in a very exposed area. I'm just showing you these pictures to give you one example of a good defensive layout. Every situation, every demoman, is different; everyone has their own ideas. Just be clever about how you distibute those pipes. Try and place traps in choke points, where the enemy will be most likely to go. The demoman is an incredible asset on defense, so exploit every advantage you can out of your pipebombs and the layout of the map.
Normal grens are Important!
It's important that you use your normal grenades (and hand grenades and MIRVs) when you've got a few traps set. They can still be very effective, and by the time an enemy makes it to the basement, or wherever you are, all it may take to kill them may just be one well-placed grenade. You never know :)
Oh yeah, don't forget to check out the offense section for some tips that can be implimented on defense as well.