The story involved a tower on earth which might have been called the "pen" or "umbra" (penumbra?), and there was some sort of anomaly inside it. Humanity was trying to penetrate a forcefield in the anomaly without success.
Someone eventually figured out that a kind of sentience was purposefully blocking them from the other side, and that if they used randomly-timed strikes on the barrier they could get through.
Read before ~2008. Possibly part of an anthology.
