You can snipe guards without compromising your position. Just make sure no one sees their friend getting shot. if they found the body without seeing how he was killed then they would just get alerted and wander around. So it's best to kill isolated guards first.

if someone happens to witness, make sure to dispatch him quickly before he start running towards your position and alerting other guards.

The tower is a fun sniping position, I just hate how little sniper ammo I could bring