To start the building you need to have 12K Ale, 15K Timber, 5K Steel, 15K Plates, 3K Thoriars, and 500 "Scrolls". It is not possible to use Gold for the upgrade. Building time for level 1 is 2 weeks and 1 day. Only the Chief or any of the Elders can start a construction process, but all clan members can send resources to the Stronghold Resource Storage.
For speeding up the building process you use a Stronghold boost.
For level 1 the stats are
Stronghold Influence: +188M
(someone else can make a proper table later)