Dujiangyan irrigation

If I wish to start my tour of Dujiangyan irrigation at the top of the viewing tower which is a different entrance from the main gate to get a panoramic view without the climb up ( which one needs to if go by the normal main entrance)--this is what I have read. However, I do not know which is the right entrance to go in by (although I know it is not the main entrance). Can anyone help with this and also which point to drop off from Bus #4 to get to this correct entrance? Thanks.