Minecraft Castle Defense System

Update: You can find an updated video here: http://www.youtube.com/watch?v=OAnZ_HLocnU

First of all: sorry for the weird colors and the weird watermark... My screen-recording program is kinda weird =S. Anyway;
This is a security system I built into my castle (it's all explained in the video, be sure to turn annotations on). If you remove a block, lava will come falling down and the ground will disappear from under you. This is all achieved with torches and gravity (gravel). I also show a small part of my city. The only things I cheated are armor and tools... All the materials were gathered.

The closing gate works as follows:
There's a tower of gravel with a torch attached. On the torch is another pile of gravel, on which there is another torch etc.
If the first tower of gravel starts to move because of gravity (i.e. removing one block) the torch disappears, which makes the gravel fall and the other torches disappear. The gate is closed with gravel.

The falling lava works like this:
It's actually pretty easy and works with the same system as the gate. Some torches are attached to the gravel that's supposed to close the gate when the system is used. On top of those torches is a block of gravel, and on top of that is some lava. When the torches are destroyed because of the falling gravel, the gravel that traps the lava falls down one block and releases the lava. It then comes down through the shaft to drop onto the bridge.

The disappearing ground works like this:
This one is a bit more complicated. You can't actually see anything of this happening, except for the disappearing ground itself. When the triggerblock is destroyed, a torch inside the wall gets destroyed. On the torch there is a block of gravel and on that there is a spring block of water. When the torch is destroyed, the gravel falls down and the water is released into a small series of tubes. There are some torches in the tube system and when the water hits them, they are destroyed. On top of the torches is once again some gravel, which is trapping more water, but one block higher than the water that's already there. The released water then flows under the gate to destroy some torches holding up the gravel and the gravel falls into a hole all the way to the bottom of the map (I'm not kidding... You can even fall out of the map if you remove a block at the bottom).

I hope this explained =P. Enjoy!
(Oh, and also I show a small part of my city by looking around...)

EDIT: I've updated part of the trap. All three blocks of the ground now fall at the same time, instead of two first and then another one.