Once you've smashed down the portcullis on yourself and survived, can your character move in either direction? Or are you trapped and immobilized by it?!
Yes, he can move afterwards, and yes I bought one dart for the sole purpose of triggering the portcullis from afar. There are a few reasons why I decided to do this, in order of priority (SPOILER):
1. I was curious
2. It had never been done before (hence the curiosity)
3. (SPOILER ALERT) In the Goblin Fortress it took me awhile to figure out this one part with portcullises that closed on you when you stepped under them, and I realized it would be possible to cheat the system if you could survive them.
So yah, I wouldn't recommend doing this, I mean it's not THAT rewarding, it's just funny to see a gate crash down on your character and the 'so and so suffers 350 points of damage' message that appears.