'Enemies become aware of your presence over time. Larger populations are harder to hide, and generate more awareness.'
The awareness meter can be found under the mini map on the base view map. This allows you to track your current awareness percentage as well as the awareness generation per day.
'When the awareness meter is full, the demons will raid your colony. As the days go by the raids will get harder and harder. To reduce the awareness growth rate you can: perform research, learn stealth skills and build & patrol Lookout Posts. Don;t let your colony grow faster than you can hide your presence or you may be overwhelmed.'
You can also use rituals to lower your awareness generation per day.
You can lower your current awareness level by defeating a demon Scouting Party.
Even if you follow all of these precautions there is a hard cap minimum to awareness generation based on the game difficulty:
Tough - 8% / day
You will have to deal with base attacks eventually.
'As time goes by, the demon infestation spreads and enemies become stronger. Later in the game you will encounter a larger number of more powerful creatures.'
The base difficulty of any base attack is dictated by the day of the attack. Contrary to popular opinion, no other modifier, such as the number of survivors, rituals cast, the level of development of the base, the number of resources, etc, effects the base difficulty of these attacks.
The base difficulty increases on a curve from day 10, with the progress of time being incrementally less impactful. For example, between day 150 and 190, the overall level of difficulty is increased by 120%. From 190 to 210, it increases by a mere 44%. The curve becomes more linear on day 210, increasing at a steady rate of around 0.5%, and being capped on day 1000. All attacks from day 1000 and onwards will have the same, maximum, level of difficulty.