Horde levels are a new game mode for Overcooked (YAY!

). The Unbread will start breaking into your kitchen demanding a certain dish (burgers, pizza, pasta, etc.). If you don't get to serving them in time, they will break through the boarded up doors and start damaging your kitchen. It's essentially the equivalent of not missing any orders in a normal level. There are a total of 8 Horde levels added into the game with this achievement requiring you beat all 8 without losing any kitchen health.
New Monsters:Unbread: Basic level of foes you will face. They will approach the doors asking for one meal and slowly work the door's health down.
Chili Pepper: These guys are faster in getting to the doors and attack faster as well. They expect only a single meal.
Rotten Apple: These guys present the biggest challenge. They are big and slow, so you have time to prepare their meal. However, once they reach the door, they do very heavy damage despite attacking slowly. The hardest part is that they expect a second meal and you won't know what it is until after you've served the first one. That gives them plenty of time at the door to do damage.
New Mechanics:Doors/Kitchen Health: The wooden barricaded doors have a health that depletes as the monsters attack it. Similar to food when it starts to overcook, the door will beep and blink with a warning as its health gets low. If a door is completely destroyed, you have a very small amount of time before the monsters' next attacks do damage to your kitchen, voiding your chance at this achievement.
As you serve dishes you will earn money which can then be used to repair doors as they are damaged. You can repair doors while they are being attacked (very useful in later levels).
Locked Pathways: In the last couple Horde levels, there are pathways blocked by a locked gate. Usually they provide a faster pathway around the level or access to additional cooking locations. They are unlocked with money earned from served dishes.
I played through in Co-op and found the first 6-7 levels very easy to stay at full kitchen health. The last level was by far the hardest. I'll provide a couple tips below.
**Unlock the rightmost gate when you have excess money (~100 more than needed). It's much better than the leftmost door.
**Prepare basic cheese+tomato+doughs when you have breathers between waves.
**Repair doors as you go to keep them at full health.
**Save up money for the last wave. You'll be faced with 4-5 simultaneous apples. It was helpful to serve both meals to a couple of them and then fend off the others with constantly repairing their doors while the pizza is cooking.