Shields come in a variety of shapes based on the cultures and time periods.
The Late Roman shield appears to be disc shaped and lenticular, mostly based on one found at Dura Europas. They have a round or oval shape.
Shields used by the Norse appear to be generally round, sometimes flat, sometimes disced.
A kite shield and the teardrop shield appear in the Early Medieval era.
There is evidence of Pictish shields that are small and 'H' shaped.