Hi!
I haven't played NCP since some time ago, but i will try to give some answers...
- To build a depot you need to have two supply wagons units in the stack (or alone in a separate stack). Click the build depot button and next turn they will be consumed to build a depot on the region. Its a fixed structure, like a city
Depots produce supply, collect and forward supply from nearby sources and send it to nearby units, allow units to quickly recover health and will also shelter the troops in the region form bad whether in winter like a city.
You can build them on a region with a city or on an empty region.
They can be destroyed by the enemy, so don't forget to protect them.
- Engineers help in sieges. Having one on a stack sieging a city will give you +1 bonus on the siege rolls. This bonus don't add up, so having two engineers on the same stack won't give you double bonus.
- Forts: i'm not sure fort building is used on NCP
You cna in WIA and AACW, but can't recall if its also on NCP.
In any case, it works like the depot building, but you also need an arty unit plus the two supply wagons. Forts work like other structures (cities, depots...) in that they will shelter form bad weather. They produce little supply but offer defensive bonus to troops inside them.
Cheers