What is "to trench at"

Trench \Trench\, v. i.

1. To encroach; to intrench.

   Does it not seem as if for a creature to challenge to itself a boundless attribute, were to trench upon the prerogative of the divine nature?
   --I. Taylor.

2. To have direction; to aim or tend. [R.]
   --Bacon.

   To trench at, to make trenches against; to approach by trenches, as a town in besieging it. [Obs.]

   Like powerful armies, trenching at a town By slow and silent, but resistless, sap.
   --Young.