you need a set of tools to unlock a door and the pressure plate is a trap so if you disarm it then thats that... of course the key is the best option
p.s, isn't it lighting the fire pits either side of the pressure plate that unlocks the door? its been a while
Tried that, no luck, and I had lots of Lockpicks on me, but it said it was impossible to lockpick that door. It also seems to be immune to all damage, as I said above.
