Its all written in l2tower forums:
How to run L2Tower on USA server [NCsoft west]
1. Download it:

2. Unpack to system folder (replace)
3. Delete GameGuard.des & "GameGuard" folder from system folder.
4. Copy L2.bin and change it to L2.exe (use control panel>folder options>view>unmark hide extensions for known file types)
5. Run L2Tower loader
6. Chose path to l2.exe
7. Click Run & inject
8. Play
It works perfectly for me.