The "Night-Siege" event can be activated by an R4 or R5 in the event center. After the weekly reset you can activate the event three times. Once activated the players can participate by using the event center. There is a dedicated map for this event, similar to the expeditions. Therefore the position of your castle on the map doesn't matter.
During the event you need to defend the Underworld Gates in the middle of the map. It will be attacked by waves of creatures, that have to be defeated by the players, which have to reinforce the 5 fortresses around the gatess. Every fortress has a number from 1 to 5 to make coordination easier:
There are several waves of creature attacks. During each wave all 5 fortresses will be attacked. Every creature that was not beaten by the players will make damage to the Underworld Gates.
Creatures from fortress 1 deal 6% damage (not sure).
Creatures from fortress 2 and 3 deal 5% damage.
Creatures from fortress 4 and 5 deal 4% damage.
When Underworld Gates reach 0% you loose and the Night Siege is over.
With each wave the creatures get stronger.
There are up to 30 waves. When the Underworld Gates have more than 0% left after wave 30 you won.
There are 6 grades of difficulty. You need to win the current difficulty to open the next higher one, so first you need to win an "Easy" Night Siege.
The Night Siege can be planned in advance by R4 or R5 in the Event Center, so everyone can prepare himself. The Night Siege doesn't start automatically, it has to be started manually by an R4 or R5.
For the event the personal fighting bonus and march capacities will be used, but troops are provided by the game. It is not necessary to have a free marching slot, so it doesn't matter if your troops are outside farming.
After a wave is announce, you can see the number of enemies for this wave. For example "Creatues left: 6". Each fortress has a predefined number of attackers:
Fortress 1: 2 creatures
Fortress 2: 4 creatures
Fortress 3: 4 to 5 creatures
Fortress 4: 6 to 7 creatures
Fortress 5: 6 to 8 creatures
Every player that is startioned in a fortress can only fight one creature per wave in that fortress. If you have less players than attacking creatures, those creatures will deal damage to the Gates.
If a player can't defeat a creature, the next player in the row will fight it afterwards. The first defender is the strongest player in the fortress and goes to the next "weaker" defender. The strength is not measured by the players power, but maybe by the march troops strength.
During a wave troops can switch between fortresses and defend different creatures of the same wave. This mainly works from fortress 4 and 5 to fortress 1, because the walk time difference is big enough to send troops to fortress 1. This way one player can defeat two creatures. Afterwards there is enough time to get back to the original fortress.
There are different types of creatures. Some are very strong and slow, other fast and weak. Fortress 1 has strong creatures that walk slowly, fortress 2 and 3 get medium ones, while fortress 4 and 5 get the fast creatures that are weak.
The most important thing is to have a lot of players. They don't need to be very strong to reach far. Farm account with 200% bonus could defend fortress 4 and 5 till the end easily. Which makes each player valuable.
It is important to have a look at the happening. If players A, B, C in fortress 1 do well, maybe player D in fortress 1 should switch to another fortress if help is needed. You have to test how the event works best for you and how you can improve.
There is a battle report to see how good your defense works. In this case creature 4 was not defeated and dealt damage to the Gates:
It makes sense to name one player per fortress to have an eye on the battle reports, so that the player can warn the rest that next wave will be potentially a lost. If another fortress can send one player that is not needed, than the player should switch the fortress.
Each wave can have 22 to 26 creature, so the tactic is very dependend on the number of participating players. If you have more than 26 players the event becomes quite static, as every player will stay in one fortress the whole event, at least if the strength is enough. If you have less players, some players will need to move from fortress 5 to fortress 1 each wave. Other changes are nearly impossible, at least during one wave. If you have less than 22 players you will automatically loose after some waves.
It would be best to have following players in the fortresses:
Fortress 1: 2 very strong players (C30+ would be good)
Fortress 2: 4 strong players (C26+ would be good)
Fortress 3: 5 strong players (C22 to C26)
Fortress 4: 7 players (C16 should be fine)
Fortress 5: 8 Spieler (C16 is enough)
From time to time a bonus creature will spawn on the map. There will be a notification in the top left. You can defeat those bonus creature for additional rewards. Everyone who is in the event will get the reward, so it doesn't matter who kills the creature.
When the creature spawns it will be protected for 30 seconds, afterwards you have 2 minutes to beat it. It is suggested to have one player per fortress who will take care of the creature. So it is easy to just name the responsible fortress and the player will take care on its own, of course after he defeated the current wave. The worst case would be that several players will attack the creature and also not defend their fortress during that wave.
All rewards are based on the overall points earned (depends on the damage dealt to creatures) and the number of defeated bonus creatures. These rewards are related to the curiosities. Surprisingly there is no big difference in rewards if you loose in wave 29 or win in wave 30. Reason is probably that the next reward level needs more points than earned. So it doesn't matter if you win or loose, only the points are important, which is different to Underworld event, where you have to defeat the monster.
Therefore it is probably better to choose a higher difficulty and loose, instead of playing "Easy" and win.
The event is something new and not based on big spenders, it is more about team spirit in the alliance and activity. Unfortunately there is no real information from the game developers, so everything is based on experience and difficult in the beginning. The event gets boring for most players probably, as you just need to place you troops and do whatever you want. It only needs a few active players during the event. It is usually difficult to have enough players, but if you have the event gets easily boring.