Is there a natural way to increase levels of testosterone in the body to aid muscle growth?

As well as what the guys have suggested, you may want to also try some Reflexology treatments.
A non invasive way of stimulating the organs responsible for the production of Testosterone, and a great way of relaxing.

The best way to do this is to do low reps high weights. Aim for 4 - 6 reps of 80 - 90% of your 1RM. Your muslces and body will adapt to this increase of intensity, increasing your testosterone levels

Yes diet and exercise and good quality sleep.

Heavy heavy weights and low reps to boost testosterone and sperm count