All of the siege weapons in the Ascalon City fractal are immune to single target ranged attacks, even at point blank range.
To duplicate:
1. Roll Ascalon City Fractal
2. Equip a single target ranged weapon, such as ranger longbow (engi pistol, ele fire staff can still hit because those are aoe)
3. Try to attack a siege weapon
4. Be obstructed at all ranges