Did you go down via the tunnels? You may have been above them, hence them showing up on track and you not seeing the tower.
As a side note, I'll hijack this bug report to report that the pathing/AI of these goblins (#14 on the map at
http://wiki.project1999.com/Lake_Rathetear) by the underwater tower is crazy. They can melee you from afar when rooted.