The presence of sufficiently large enemy fleets at a colony will limit what a colony can do. A Larger colony will require larger fleets to be blockaded.

A colony will be blockaded if there are large enough enemy fleets at the colony. The fleets need not be from the same player nor does any of the fleets need to have a certain minimum size, as long as their combined power is enough.
Only ships with offensive weapons count towards a blockade and only if the owner of the fleet has set the owner of the colony as worse than neutral (currently only enemy is worse than neutral).

• Buildings can no longer be destroyed or built.
• Ships and ground defenses can no longer be constructed.
• Ships should be unable to be bought or sold on the market.
• The colony can no longer disband.
• Immigration is severely lowered.
• Emigration is severely lowered.
• Happiness is slightly reduced.
• Crime is slightly increased.
• Entrepreneur income is severely reduced.
• Retail store turnover is severely reduced.
• Resource production is severely lowered.
• Blockading player gathers intel on colony military assets.

All blockading fleets collect intel on the colony.
Each game day your intel resolution increases by 30, 10 if the colony is under Martial Law.
Each day you are not blockading the intel drops by 2.
At 100 intel you learn the military index of the colony.
At 300 intel you start to learn how many ground defenses the colony has. As you gain more intel the reported range shrinks.
At 500 intel you start to learn the average tech level of the ground defenses.
This intel does not include civil defenses but you can estimate this by the military index and the population.