How to Play Defense on an Attack/Defense CP Map

This seems relatively simple ostensibly, set up a few good Engineers, Heavies, and Demomen, and you should be fine. However, there are a bunch of nuances that can separate victorious team from dominant ones.

Engineers play an extremely important role. However, unless they can set up camp, they are useless. Therefore, having a combat class stick by an Engineer while he builds defense for himself gives him something to make sure he doesn't get overrun, and therefore allows him to do this quickly. This is extremely important when, after you lose a point, he tries to roll back to the next point for the taking. The quicker this can get done, the quicker you can create a bottleneck.

There are two stages of a defense: Rush and Bottleneck. The former is when he other team seems to be attacking points with relative ease, and the latter is when defense is in control. Even though the latter seems like a good place to be all the time, a good rush can allow for a powerful defense. If you decide not to defend a certain point, you can use this to give time for a buildup of Engineer nests in key places which are difficult to ascertain in a quick rollback. Also, it allows other classes to get into places which make them the most useful.

The key classes in defense other than Engineers are Medics, Demomen, and Snipers. The latter are only useful if they are good, as they can stop any class with ease with a well placed headshot. Two good snipers can stop any offensive in its tracks. The Spy can alternatively do this, but it's much more difficult to perform this as Spy. Demomen can spam grenades around corners and down checkpoints, laying down suppressive fire which will force a team to not move. Since respawn times near 20 seconds all the time, Medics become very important in this style of play. If you can heal classes you can't afford to lose, you give them another life within which to perform their duties.