From Halopedia, the Halo wiki
This spacecraft launch station is circular in shape with three main levels. A fourth level is accessible via Jet Pack, but only for ten seconds at which point you will be prompted to return to the map. Countdown is a small map with only three power weapons available here, the Energy Sword, the Concussion Rifle, and the Shotgun. These weapon's locations will be highly contested areas.
The Jet Pack is a great choice here. The stacked, multi-level structure almost requires you to use one. Get to upper and middle tiers before enemies and without having to use the gravity lifts where enemies might be waiting for you. The Hologram is a good tool on this map as well. Firing one off into the center of the map will always attract attention in enemy areas.
The circular layout creates a constant flow of play-keeping players moving throughout it at a fast pace. Use gravity lifts to quickly change elevation and evade enemies. Under the ship are three gravity lifts you can duck into during a battle if you need to recover some health. There are also two lifts located in the inside corners of the central portion of the level.
Beware of enemies camping at the gravity lifts just waiting for you to pop up so they can stab you. Toss a grenade or two up the lift if you can before you jump in. If you are so equipped, try sending your Hologram up the lift, then while the enemy is busy with it, toss a grenade up and finish them off. Camping the gravity lifts is a little cheap, but can get you some quick, easy kills if you are in need. Use the Energy Sword or Shotgun for best results.
Most of your engagements here will be at close quarters so prepare for that kind of battle. Think about Armor Lock as your Armor Ability, and don't forget to melee. Don't worry too much about trying to Assassinate players here since while the layout is stacked vertically, it is very open, similar to a stadium. Performing an Assassination that everyone will see will attract attention and focus all eyes on you, possibly resulting in someone stopping you in mid-assassination.
The level's name is a reference to the setting, as ships undergo a countdown before being launched.