We have here a crazy fun map filled with scripted events to change to the gameplay on the map.
Here are some key features of "Icehollow":
On the map there is a stationary Flakvierling cannon. You'll find you can't get to this, because if you shoot near at it, it's surrounded by a field.
Self explanatory, use these to traverse the map at high speed. It uses a fixed velocity, so you won't use it to jump out of the map.
This can be found in the L-shaped house which has a corner broken off it, surrounded by another field. Press the glowing switch, jump into the rings, and wait. You'll get to choose where you want to go in a room which gives you the option.
People have told me this is the maps' best feature, I don't argue. Setting it off is more difficult, as you have to look in the right place, at the right trigger. Go to the house with the computer in it and look at the inside corner of the desk. There is a trigger_look at hidden there, which will make it obvious when it has been set off. Now look outside.
This part isn't done yet, but will be in the next version if I can be bothered. It teleports everyone to a large flat area underneath the map, where there is a FIGHT TO THE DEATH! Then stuff happens and the map goes on as normal. All a bit vague, I know.
There's a map-based turret in one of the bunkers, go up to it and press E to use. It doesn't add to your score when you kill someone, as the beam goes through walls so wouldn't be fair anyway. If you're being shot by it, then go prone. It can't hurt you there.
In the house that has the ion cannon trigger, there is a trigger_use stuck on the wall somewhere, just after you come up the stairs. It retracts the stairs for a few seconds, so no-one can get up, then puts them back.