I love starting out in such places. I haven't ever started direcly in the middle of them like that, but I like to pick places with them, and the stray goblins have often spotted me all too quickly. I like to immediately dig a dual-level mote (simply the height is enough to prevent things from getting to my wagon supplies) around my wagon, and dig into the earth. Once indoors, one can strategically dig under rivers to begin one's farming operation, and begin hauling everything to store rooms. Later, a drawbridge is built, which is only lowered when merchants and newcomers arrive, if at all. It's the hermit approach to survival. I find I can get a lot done by digging deep below the fortressed, and making alternate entrances while keeping drawbridges up, such that they bring the caravans near enough to the enemy fortress that they take care of much of it themselves. Always remember to have traps in your cooridor, though: one time they noticed my alternate entrance and I, with still 7 dwarves lacking in any fighting skills, was quickly besieged by several goblins, which killed that digger. They didn't get any more, as the cooridor was long, and I managed to get a wall built to block it off before so they could not proceed anyway. The caravan just used the bridge that day...