I've built a PvP arena in frontier - anyone can check it out. From the top, in the spectator stands, it looks like this
and when you're in the 'ring', it looks like this
See all those pressure plates on the floor? Step on one, and you get this
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.
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.
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.