Sentence:  ... and having a fair <WSD>loophole</WSD>, as it were, from a broken hole in the tree, he took a sure aim, without being seen, waiting till they were within about thirty yards of the tree, so that he could not miss. 
Meaning of the word marked with <WSD> tags:  A slit in a castle wall; today, any similar window for shooting a ranged weapon or letting in light.