You should put floodgates across your entrance, and link them to a lever far back in your fortress proper. You can leave them down until attackers show up. When you have seigers, then you pull the lever, and you can defend as wide a hallway or entrance as you like.
For the record this doesn't work in the 31 version, if that's the one you're using. Goblins come with Cave Crawlers and Trolls now, which are both building destroyers.
Not to start with, they don't. At least, I haven't seen the first goblins show up with any such help. By the time they do, my traps guarding the entrance have been replaced with large iron serrated discs (2 per trap, minimum), and that takes care of it.
But if I was going to ultimate defense, I'd build a 2 tile wide moat, and put a couple of draw bridges across it. 1 is only there to allow dwarves to cross out to get wood and plants while the main bridge is first being tested up. After that, it stays withdrawn for the rest of the game, unless needed. I had several main bridges destroyed in version 0.28, so that's just a habit of mine. Always have a backup way across your "impenetrable" moats. Although with the richer underground in 0.31, I suppose that isn't as vital as it once was. I like to have levers inside my fortress for controlling the bridges. That way, you don't have to worry about having someone run out and pull the lever and getting scared away from the job because of what is nearby.
What version of the game are you playing? In the 31 series the metal is abundant. In the 40 series (which came BEFORE the 31 series), metal is relatively scarce. I'd look at the wiki for 'exploratory mining' if playing 40. I'd just dig out a base normally if playing 31.
OP is using 28..40d from what has been said in the thread.