I've built a PvP arena in frontier - anyone can check it out. From the top, in the spectator stands, it looks like this
http://dl.dropbox.com/u/80858415/hosted/arena1.jpg
and when you're in the 'ring', it looks like this
http://dl.dropbox.com/u/80858415/hosted/arena2.jpg
See all those pressure plates on the floor? Step on one, and you get this
http://dl.dropbox.com/u/80858415/hosted/arena3.jpg
You'll see pistons fire that change the landscape just a little bit - enough to upset an archers aim, and a dispenser in each corner will fire an arrow or two. The best part is, some of those plates are fake - they do nothing - and the ones that are wired will fire based on a random number generator. Yep. Under the floor is a computer that outputs a random number and that will fire one, some, or all pistons/dispensers.
Here is the underground computer/wiring.
http://dl.dropbox.com/u/80858415/hosted/arena5.jpg
Scary stuff but a learning experience.
Anyone can have a poke around - make sure both players are 'ready' - that is both ready switches are down. Yup - the computer even has a master switch requiring both levers to be down. Here it is on Frontier.
http://dl.dropbox.com/u/80858415/hosted/arena4.jpg
Now this isn't spleef of course, but it took me probably 5 days to get that computer working with all the wiring. Happy to offer any knowledge that might be useful.