The soldier defending the tunnel has the easiest job of all. He has a nice narrow access to defend, which makes splash damage from his rockets easy. Just watch out for the NME concs. If the enemy does manage to get to our flag room, he should watch out for them trying to jump out of the corner of the pit into the yard with the flag. This map is an attacker dream.
Speed is essential.
Scouts are great for concing yard defenders out of place and running through the sides of the flag room and back up the stairs before anyone even sees them. Medics are also good at this, with the added benefit of being able to take out sg's and stuff with thier grenades. The fourth guy should concentrate on clearing a path for the other attackers, and use whichever class suits the situation best. Spys work well if all the defence is concentrated in the pit area. Soldiers work well to take on yard defences while teammates slip through behind you.