A couple things:
"Xuenylom" -- this challenge is a tribute to Populous and Peter Molyneux. The title is Molyneux spelled backwards (pretty obvious).
A trick for "Drill" is to send your guy right away for the knowledge, then quickly build the wall, then drain the lake -- in my case the guy was on the far side (near the knowledge), which meant he was going to have to walk ALL the way back around and down the slope. So I picked up some water and washed him into the canyon (trick from the earlier challenge). That will probably save you some time.