In the character class rules section it's told that Fighters get an additional attack every 5th level after level 15.
That's stupid, because several levels earlier all characters already have an option to get stable with own castle, stronghold, wizard's tower etc.
And other classes have these other knick knacks they can perform while fighters only swing swords.
That's why fighters get an additional attack every fifth level starting from level 5.
Opitonal: instead of increasing # of attacks Fighter can optionally gain another d20 when rolling to hit (# of hit dice). The best (highest) result of these additional dice is kept.
11th level fighfer could have:
A) 1+2 attacks per turn
B) 1+1 attacks per turn and second hit die
C) 1 attacks per turn and two bonus hit dice