Hard to hit with arrows

There were places where the wall still stood , secretly maintained in a way that ensured it didn’t look maintained . Vertical slots lined the stone every five yards . Long ago , those slots allowed archers to launch death down upon advancing troops . Now , though , the slits were useless — demons were so fast they were hard to hit , and a single bolt barely slowed them down .

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Aliens: Phalanx, Chapter 2