I was playing on a frozen map when it occured to me that I could actually have running water (due to magma) and pump it into my entrance tunnel where it stayed liquid. (clearly I was low enough down for that). However I'm pretty sure if I open that tunnel to the air it would all freeze.
So here is my defence plan.
First off a line of pressure plates at the entrance which trigger a gear connected to a pump to fill a square of water. This square of water is on other plate and so produces an always on signal once any hostile triggers the entrance no matter that it'll flicker on and off as people travel over it.
This second plate will be connected to another gear/pump system or maybe just a flood gate which will fill a small room, time to allow hostiles to get a fair distance down the hall way. Once that is full a 7/7 plate will open a gate shifting all that water into the hall way.
As the hallway is ramped at both ends it will fill up to 7/7 quickly at which point a plate at just off the far end of the hall, which is covered by a roof, triggers both a delay pump/room combination and the trapdoors/bridge/whatever that covers the hallway. All the water then freezes killing the hostiles.
Once the last delay pump/room is full it triggers magma to fill the a hall below the hallway and a floodgate to drain out the water, finally this magma is allowed to drain as well and the trap is reset.
Does anyone see any major flaws in this, other than the fact I will obviously miss something and end up pumping magma into my living quarters or something.