There were maybe underground tunnels under Orthanc Gandalf though that. That's why he said to treabeard to keep the water in Isengard. But when Gandalf was away Treabeard released him cause he could do nothing anymore cause his staf and wil were broken.(that's wat treabeard thougd).

For materials I should use plywood ore MDF for the base of the building but I should build orthanc seperate and a board with the wall of isengard otherwise it is to heavy. The tower I should build out of MDF and use plastic card for the details.

hope this helps.