The hill is Golgotha or Calvary on which Christ was crucified. See John 19.17.-42 for one of the accounts on Good Friday.
Litotes, meaning 'alone'
'we' are the three crosses: that of Christ and those of the two thieves crucified with him.
Helena, mother of Constantine, and Saint Cyria discovered the Cross in the fourth century.
This 'undertaking' refers to the Harrowing of Hell, wen Christ rescued the souls who had been condemned to hell following the centuries after the fall of Man. This apocryphal event took place in the days between Christ's Crucifixion and Resurrection.