remember, if you want more space you could dig a new entrance and wall off the old one. For a flooding chamber you really need a z level below to dig out the drains. Traps doors and draw bridges are fine against goblins, but when something with the building destroyer tag shows up you either get lucky with cage traps (cage traps are way too good) or you need a well designed flooding chamber where the creature gets drowned before it can destory the drainage hatches/flood gates for the room or break out another exit.
another cheap option is to build animal lures. Build some pillars around the edge of your map (a few tiles from the edge, 6 maybe) and chain an animal to each pillar. Then build walls around the animal leaving one entrance (if the goblins or megabeasts cant reach the animal they wont go for it, but walls are useful to stop archers from just shooting it). Then surround these walls with cage or weapon traps. This will both kill goblins and slow their advance giving you time to get all your dorfs into burrows. An improvement on this is to build what is known as the cave in trap. Dig out a z level below a small hill, build one support, build a chain attach animal, surround the animal with triggers, all linked to the support. When enemies close for the kill they trigger the support to colapse and bring down all the z levels above on the intruder.