Two lighted lanterns hung from pegs along the center of the stable, and Callie had mounted a barrel to put up a third as Drew entered.
Callie mounted the barrel to hang another lantern, but then fell down. 
mnli hypothesis: Callie mounted the barrel to hang another lantern, but then fell down.  premise: Two lighted lanterns hung from pegs along the center of the stable, and Callie had mounted a barrel to put up a third as Drew entered.