Spearmen attack port?

[HRE_HeinrichV]

I just saw some danish land units, of an unknown type, blockade my port. the had retreated from a major fight with my haggard northern army, and then fell upon the port, between my castle and the army. I didn't know this was possible.

[lars573]

When an army moves over an enemy port on the campagin map it blockades it.

[Mr Frost]

It is a feature carried over from RTW .
If an enemy unit stands on one of your roads it will start to choak off the trade flow on that road also . Best to finish off any hostiles in your territory quickly as devistation is sometimes the least of the financial cost if you let them live .