Drake will fall through the floor, and wind up in a cave below. Before you move ahead, look behind a few stalagmites that are off in a corner. There's a treasure that you can pick up sitting behind a little grouping of them. After that, it's a fairly straight walk forward though. Just move up and jump into the water ahead, then dive under the rock to wind up in a narrow tunnel. If you keep moving forward, you'll wind up in a large circular chamber, with some ledges that you can climb up to reach the old stone well.
Things are going to get a little tricky though. Marlowe's boys are here, and it's time for a gunfight. Start things off by doing a stealth takedown on the thug sitting by the well. You can then hop out and grab his gun, and take up a good position behind the well. From here, you can pick off a few of the mercenaries in the yard, and then watch both sides for flankers. If anyone tries to get around you, either hop out of cover and use melee attacks, or use blindfire to shoot in their general direction quickly. At close range, you'll be accurate.
Once the first wave is taken care of, move forward to hit the stragglers and find a few more gunmen up in a window. Get by a wall and pick them off so that you can climb up and go through the newly opened windows. They apparently left behind a nice cache of grenades, so pick a few up and go through the grenade tutorials to take out the next two groups and reach the main courtyard. After taking out the first group, you'll be able to drop down and reach a nice weapons cache. Stock up on ammo, and look for a piece of treasure in the corner before you move on to the second tutorial.
Sully's just ahead trying to hold off the mercenaries. You can start things off by going up the ramp to the left and reaching a new grenade stash. Go ahead and toss a few out to kill a couple of the thugs, then start shooting with your pistol. Don't get too invested in this spot though. Two men will kick in the locked door below and charge up the steps. If you're quick you can melee them both while they're on the stairs, which should result in a weird high kick attack that will take them down instantly.
Go out the gate and see if you can join up with Sully to the left. There are a lot of thugs spread out around the area. Thankfully, they drop a lot of ammo and there should be a Mag 5 revolver in the corner. This will kill anyone with a single hit, so it's a good weapon to have. It's not vital to surviving though. Just staying by a stone arch and trading fire with the gunmen should be enough.
After a second wave hits, you'll be able to go up a few sets of steps and find a door that you and Sully can knock down.