Inside the castle where you plant the bomb upstairs there is a wooden rafter use that to quicksand inside the castle. Also this is a quicksand spot but there is something else you can do in the castle go upstairs and go to the door that leads outside but stay inside, side jump onto the wooden rail the side jump to get onto the wooden wall upstairs

I didnt know you could do a quicksand in Socom 2. huh.