Name says it all. I'm currently building a Sandbox varient designed to support a unique 'tower defense' gametype. Basically, it's on Sandbox open. The defending team spawns in the centre of the map, inside a tower that I'm trying to build as high as is practical. The tower is a veritable fortress, and gaining access won't be easy. A ring of chain-gun Warthogs, rendered immobile, provides effective close-range defense against air vehicles. At the very top of the tower is a Spartan Laser, accessible by teleporter. Only one laser will spawn at a time, and there is only enouch room for one person, who will block the teleporter so no one else can get up.
The attackers have to survive assault for long enough to get to the tower, and get inside the bottom area. From there, they plant a bomb next to a pipe. The pipe is filled with fusion coils, which will set off a chain reaction which heads to the top of the tower; countless fusion coils are hidden under the floor, making for a very satisfying fireworks display for the winner.
The defenders can't escape from the tower. Or, they shouldn't, because they cannot get back to their weapons, and so will be utterly destroyed by the attacking team.
The attackers spawn behind a barrier, and have access to a variety of weapons and vehicles. The vehicles they get will be:
2x Hornet (haven't decided yet whether to use normal or Transport)
2x Warthog, Gauss
This large variety of vehicles lets the attackers choose whether to take the high or low road, so to speak. This is pretty much intended to be an arcade/practice gametype.