What counts as a "structure"?

In the fourth or fifth mission, the Heroic objective is to win without building any structures. However, you start with a single Gristmill and 4 farms, so getting food is pretty slow going.

There are a handful of unclaimed Gristmills around the map, but claiming them is essentially rebuilding them, so I'm not sure if this counts as building a structure. Additionally, am I allowed to create farmland?

I'm under the assumption that creating Warrens for units counts as structures, I'm just unsure what all is considered a "structure".

I remember doing that mission few weeks ago so this comes from personal experience. You can actually claim gristmills and build farms! The only things you can't build are warrens and the Bullet Hive ( but also defensive structures in general ).