Explanation of Unsurroundable
It does not make units immune to harassment. But it does MITIGATE harassment.
The number rating of Unsurroundable is how many time you have to be attacked before harassment starts ticking up.
So, normally every attack, hit or miss, reduces your defence.
A unit with Unsurroundable (2) has to be attacked 3 times to reduce its defence; twice to cancel out Unsurroundable, then a third time to cause a harassment penalty.