The best skills tree for Ethel to effectively and comfortably handle Barbarian Forts is shown in the image below;
Who is Ethel (Aethelflaed), and why this build?
The commander referred to as Ethel in this article is named Aethelflaed in the game. She is a legendary archer commander and one of the first legendaries you can obtain for free — which is why she is the most common early-game pick for farming barbarians and forts.
She has three talent trees: Leadership, Peacekeeping and Support. The three coloured branches in the screenshot above correspond to these; the number on each node (for example 3/3 or 0/2) shows how many points were invested in that talent.
For barbarian forts the decisive tree is Peacekeeping. Two talents stand out: Quick Study grants 15% bonus experience whenever you kill barbarians and other neutral units, and Trophy Hunter adds a chance to drop a resource pack (1,000 food, 1,000 wood, 750 stone, 500 gold). Ethel's slow effect also pairs well with this build, since fort troops melt before they can strike back.