Tactics for the Stronghold.

That's correct. That's why there isn't a 30% defense bonus, and they've been more or less balanced.
With equal stats in a vacuum, an attack expert (30%) is better than a defense expert (15%). For example, 1 angel with attack will hit 1 angel with defense for 55, and the response will be 50.
If the hero with armor has a primary defense skill of 3, and the attacker has an attack of 0, both angels will hit for 50.
For armor to be advantageous, the difference between the second angel's defense and the first angel's attack must be greater than 3.
For example, if the attack expert has 5 attack, and the armor expert has 9 defense, the attacking angel will hit for 49 and receive 50.
The higher the primary defense skill, the more advantageous the secondary defense, and the same applies to attack.

P.S. Behemoths on the third level are a bit late. ;)