How do I build and upgrade my Stronghold faster?

Four levers to speed up your tier climb:

1. Keep your builders always busy (and take the Lifetime Third Builder if you can).
2. Stack Efficiency (E): the game's formula is `cost = 1000/(1000+E) × original cost`. Every source of E cuts both time AND cost (see the Efficiency article).
3. Store your resources: level your Storehouse so you don't lose production above the cap, and level production (Granary/Lumber Mill/Quarry/Foundry) to have the wood and stone you need.
4. Ride the "Upgrade Stronghold" event for reward points while you're progressing anyway.

The constant goal: Tier 10 before the campaign deadline.