Choke Points

1. Front of Base - Beware of Snipers. They love to park themselves on the balcony or under them in the shadows. Where do they aim? Typically your balcony (to catch enemy Snipers or people trying to jump off) and the bridge. Learn to tip-toe around those dreaded red laser sights! If you do get through, be sure to lob a grenade or too up onto the balcony. If Snipers are especially bad, try going through the water. Unfortunately, this route is long and might not be any safer: you can be easy grenade bait trying to get out if someone spots you.
2. Ramp Room - Great place for a team to hang out in. It's not uncommon to find a Sentry, a Sniper, a Soldier, and a Demoman or two guarding the ramps. Their job is quite easy. Supplies are just around the corner, and the enemy has to travel up narrow ramps to get to the second floor. Tip: get them to come down, by lobbing up nail grenades and whatnot.
3. Basement - If you're lucky, this place will be deserted. A lot of players think defending in the basement is boring, since most of the enemy seem to die before reaching them. However, it's a good idea to expect at least a Sentry gun and one or two players going upstairs from the bottom respawn room. If you're attacking, at least try to get the key out of the long hallway. (Easier said than done, especially if Sentries and Soldiers are nearby) If you're defending, take that as a tip. Enlist some Engineers and Soldiers. :)

Class Issues

1. Scout - Use the waterways. In the courtyard, you're just going to get smashed by enemy players exiting their base. Unfortunately, there's not much you can do once inside. To get to the flag, you have to get through some rather tough choke points. Once the flag is out of the basement, however, it'll be vital that you keep it alive.
2. Sniper - Popular spots: balconies and in the shadows under them. Watch out for other Snipers and rocket jumping Soldiers. This is a level where good Snipers can really stop the enemy offense cold.
3. Soldier - Rocket jump, rocket jump, rocket jump. There are several places you can RJ: up onto the balcony, from the lobby with the grated ceiling, in the corridor near the waterway stairwell, and into the second level lift from the ramp room. Watch out for Sentries, however. At the very least, you should be able to distract enemy defenders for a bit.
4. Demolition Man - A detpack placed on the bridge should be able to blast players on the balconies of both bases. A great way to get rid of Snipers. But tough to pull off, unless the Snipers have horrible aim or are too busy to fire at you. The ramp room is a naturally great place for Demomen to hang out. Kills are easy, when you're raining grenades down onto the heads of your enemies. Finally, pipebombs are great to use in the flag hallway. Just don't hit the trigger early.
5. Medic - You'll probably be healing players more than usual, since both 2fort maps use items for resupplying, not backpacks. (Health boxes take longer to respawn than the packs) The narrow hallways of the level also make it somewhat easier to infect foes.
6. Heavy Weapons Guy - Defense! Park yourself in the basement, somewhere near the flag. Have the enemy come to you. With your back to the wall, they'll be unable to employ the usual circle-strafe tactic. You should at least give your teammate enough time to help demolish the threat.
7. Pyro - Injure players before they can enter your base. Fire at them when they're in the courtyard. If on offense, toast the ramp room defense, as they're most likely to be heavily injured already. Also, use the flames to annoy Snipers. A great place to hang out is in the entrance to your base. Toast any enemy entering (check for spies too), and its okay if you don't kill them right away. Just injure them enough so they'll be easy meat for your teammates. A quick kill is always better, but don't leave your position to go chasing after them through the base, especially since it may be the point of a much larger assualt.
8. Spy - Enter through the water if possible. You'll be a suspect if you go over the bridge ... most players who enter the enemy base don't make it back alive (nor would they want to). And there are Snipers who will be more than eager to report such unusual actions to their teammates. Once in the base, concentrate on getting rid of Sentries. They can be especially annoying in a level like this, where players are likely to be heavily injured once they get to the basement. Also, you can cause extreme havoc in the small resupply room. Just throw a hallucinogen and block the door ... gibs galore!
9. Engineer - Sentry placement is extremely important here. Lots of spots to put them. Most important: near the flag, to catch Spies. If rocket jumping Soldiers are especially bad, place one near the balcony or in the second level. That should deter them somewhat. Also, be sure to get a dispenser in the upper ammo room.

Things to Remember
Remember that there are TWO entrances to the basement. One through the ramp room, the other through the semi-spiral stairwell. If you're defending, learn to listen for enemy "grunt" sounds. It's a result from falling off the top of the elevator lift. If you're attacking, try to separate your offensive team. One group should at least be able to distract defenders and take out Sentries, in time for the other to arrive.