Wenn es möglich gewesen wäre, den Turm von Babel zu erbauen, ohne ihn zu erklettern, es wäre erlaubt worden.
If it had been possible to build the Tower of Babel without climbing it, it would have been permitted. [Kaiser/Wilkins]
If it had been possible to build the Tower of Babel without having to climb it, that would have been sanctioned. [Hofmann]
This aphorism is a model exercise in baffling pious argument.
There's no ban on building, even on a grand scale. The problem with the Tower wasn't its construction or even its height, but that it entails a misconception, like the cage going in search of the bird.
In the usual interpretation of the parable, the Tower is a blasphemous attempt to rival or to reach God, and man is punished for this presumption. Kafka doesn't present an opposing interpretation, he qualifies the existing one in a way that utterly shifts its footing when he suggests blasphemy arises wherever God's presence is mistaken for a barrier or a distance.
The task, like sweeping the leaves in the fifteenth aphorism, is not to take the path but to find it and keep on finding it. This is analogous to building a tower without climbing it.