Tower should shoot only in 1 direction

[fluffi]

Hi,

For example:
I have 3 difference lanes (small paths) that lie next to another.
When i build a tower in lane 1 (to protect the lane from creeps that walk along that lane), the tower will also shoot at creeps in lane 2 and 3 (if they are within range).

The tower should only shoot from south to nord or only within a area of 30 degree.

How can i prevent the tower to shoot on creeps on the other lanes?
Does anyone has an idea?

Thank you!