Epic Minecraft Defence System

EPIC!

make a big row of despensers about 10-15 long(don't curve it like I did)

You need a lot of arrows and then put them in the despensers

Put 2 powered rails then 1 detector rail for the length of the defence system, next, put 1 powered rail at each end with a block behond it
    (this makes the cart go back and forth).

Then, put redstone wire down to the despensers (each powers 3)

Put a redstone torch down for each 2 powered rails.

 

At one end of the rails, next to the block, put a lever instead of a torch by the last powered rail (this is your on/off switch)

Put a minecart down and push it to get it going.

Now it works.

Another thing you can do is place it in a giant square and put castle walls on top of it.
Then, make a circular track with the same functions.

Smaller model shown below.