The idea of crenelations is a fun one, but it happens way too late in the game to make a significant difference.
In general, garrisoning units in your castle for extra firepower isn’t worth it. Garrisoning in Towers is worth it, but because Teutons lack Bracer, and have above average castles, they’re punished for building Keeps in the Imperial age. They could build Bombard Towers, but they don’t benefit from Crenelations either. And Infantry give a mere 5 damage, the equivalent of an un-upgraded crossbowman, which is even more useless in the Imperial Age.
So the bonus is essentially useless by the time it’s available.
However, if the Teutons got it as a civilization bonus instead, then they could actually use it in the earlier parts of the game for powerful M&A/Tower combo rushes. They wouldn’t be great at it, because they lack a significant early eco bonus, but it would become a viable option.
Seeing as this would make their list of bonuses rather large, I’d propose combining the various garrisoning bonuses into a simple single one:
Teuton buildings garrison 2x units, and garrisoned infantry fire arrows.