Deployable Defences

[PBI]

What's the criteria for the option to have deployable defences to appear? I had the option for defences in one game but now the option never seems to be there..

You have to be defending, and I think your army has to have either not used up all its movement points, or else not moved at all, in the preceding turn. I generally seem to get them either when defending a city, or else defending against an enemy sally.

what tech/s unlocks fougasse?

My Jaegers seem to have them by default, I think they might just be available to any high-level light infantry.