In this map, the two teams compete to get their bomb to the enemy's base before the other. The bombs are on carts, which run on separate tracks from each team's base to the other. To move the cart you simply stand near it and fight off enemies as you push through the different obstacles along the way. In order to be successful, some of your teammates will need to stay behind to defend your base while others make sure the cart keeps moving along.
The main obstacle in this map is when your team will need to push the cart through the enemy team's base. This part of the track is on a hill, which means if you stop pushing the cart at any time before it reaches the top, it will roll back down to the bottom. This is the most common place for engineers to defend with their sentry guns, and is also near the primary exit from each team's spawn room. In order to advance through this section, you will need to clear the area of enemy players, then push the cart through before they can regroup. This is where most games will remain at a standstill for a quite a while.
The second obstacle in this map is the elevator, which carries the bomb cart up the the top of the enemy's base, where it will explode. While on the elevator there isn't much room to move around, which makes you a pretty easy target for any nearby enemies. Right next to the elevator is a platform coming out of the enemy team's base, where it is common for enemies to set up sentry guns and shoot sticky bombs onto the elevator from. Like the previous obstacle, if your team stops pushing the cart before it reaches the top it will fall back to the ground.
You won't be able to push the cart through the enemy's base while they have sentry's guarding the hill. Your first priority should be taking those out. One commonly-used, but rarely successful strategy is to have a medic ubercharge a heavy or soldier, and have them fire at the sentries from the bottom of the hill. This doesn't work too well because the long distance between the player and the sentry reduces the amount of damage you do, and it also allows the engineers to hide behind shelter while repairing their sentry. In order to make this strategy useful, you will need the help of a spy to sap the sentries and keep the engineers distracted, while you climb up the hill as you deal damage to the sentries. This will allow you to take down the sentries much quicker, and move in to take out the engineers right after.
If you can't manage to get a spy to cooperate with that plan, you can also attack the engineers from behind by going up the stairs at the bottom of the hill. This works very well if you have an uber, as you will be much closer to the sentries, the engineers will have nowhere to hide, and you can quickly proceed to guard the enemy's spawn as your teammates push the cart up the hill. When using this strategy, you will be very close to the enemy's spawn room exit, which could cause some problems if you aren't careful.
For the final push on the elevator, you'll see enemy players setting up sentry guns on the platform near the elevator, as well as demomen throwing sticky bombs onto the elevator and along its path. The rest of the enemies will generally attack from the ground. In most games, this will not be won without a lot of teamwork. The general strategy here is to first take out any enemy sentry guns overlooking the elevator, then try to take out the demomen to clear the elevator's path. At this point some of your teammates should jump onto the elevator, while the others fight off the remaining enemy's on the ground.
There is a little hiding place on the elevator, big enough only for one person. If you stand on the side of the elevator opposite of the enemy's base, near the wall, you can crouch down to stay out of sight from enemies. Your teammates should be advancing from the side of the elevator you're now on, so the enemies should have to go through them to get to you. You're biggest threat here is demomen who can launch sticky bombs onto the wall near you.
The main goal for you, while the enemy is trying to push the cart through your base, is to prevent them at all costs. When placing your sentry, try to use the crates for cover so the enemy team won't be able to shoot you while you're repairing it. Putting it on top of the crates is not a good idea because of the reduced range of your sentry, and snipers will have a clear shot at you while you're repairing it. Build a dispenser behind you, around the crates, where it will be safe.
If the sentry guns are down, and the enemy team is pushing the cart up the hill, you can jump down and touch the cart yourself to make it stop. Even if you can't kill them all, you might manage to scare them away from the cart far enough that it will roll back down the hill. This will buy your team a little extra time to regroup and move in to help defend.