how does enemy expansion work?

[mcwaffles2003]

I have my entire base walled off with dbl thick layer of walls and turrets closely lining the whole wall. Can someone explain why expansion chunks are shown inside my walls in the yellow regions? Id figure they couldnt expand there if they have no path to reach there

[torne]

First the enemies decide where to expand to, then they try to path there, and if they are blocked by your walls they will attack the walls to try to get to where they want to go. So, as long as you have turrets defending the walls, they will just get killed on the way.

Only proximity to player structures (and pollution?) is considered when deciding whether to expand into a particular chunk, not whether they'll be able to get there.

[Bushdoctor]

It is important to realize that aliens do not spawn in a new spot. They have to separate from their existing pack, physically move and settle somewhere else.
So, even though the map shows that a certain area is suitable for expansion, if your defenses hold up they won't be able to reach that spot.
(I realize that wasn't a direct answer to your question, but it cleared up a lot for me when I found out)

[mcwaffles2003]

Thank you both, those green circles had me worried.
Cheers