# Alien Invasions

Every once in a while, a swarm of hostile aliens will storm the Lands. You have to prepare in advance to hold them off and protect your base. Aliens come in waves, and there are various [types of them](/game-systems/combat/aliens.md).

You will be notified about the upcoming, ongoing, or finished attack whenever it happens. Aliens can invade while you are offline.

### Frequency of the Attacks: Security Rating <a href="#threat-level" id="threat-level"></a>

The frequency of the invasions is determined by the [Security Rating ](/assets/lands/land-security-rating.md)of the Land.&#x20;

### Strength of the Attacks: Threat Level <a href="#threat-level" id="threat-level"></a>

The Land’s **threat level** defines the frequency and power of each alien attack. **Threat level** is generated by structures and extracted resources.

* The amount of threat generated by each **structure** is defined by its level and type.
* Any **resource** present on the Land, raw or refined, generates threat, including resources within Warehouses, [Miners](/assets/units/resources-miner.md), [Mobile Depots](/assets/units/mobile-depot.md), etc.

Units do not generate threat, so upgrading the units present on a land or allocating more units there are effective strategies to counter alien invasions.

### Alien Behavior During Attacks <a href="#alien-behavior-during-attacks" id="alien-behavior-during-attacks"></a>

Aliens enter a land through the **land’s entrances** - passageways that communicate it with other lands.&#x20;

When aliens have valid targets within their perception range, they choose targets according to their gameplay. When they have no valid targets within their perception range they **target the highest source of threat** and move towards it.

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Once the threat level drops to a certain value, aliens seem to consider that their incursion has been successful, and leave the Land through the nearest entrance.


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