Hill altar is a persuasive term but - is there any evidence that things happened on them ? It's also possible that they were admired from nearby. Another question is whether the artificial hills were copied from natural ones - particularly drumlins - from landscapes with a different geology. Judging from the three I know best, Proudy Hill, Sugarloaf Hill and the one near Thornhope who's name escapes me, they were surrounded with other monuments, large and small.