To maintain a constant pressure and temperature, without 
any variation in the whole fuselage or ease of circulation 
of constant pressure and temperature the fuselage is built 
in circular cross section.

the fuselage of the pressurized aircraft is made circular
because since fuselage is an aerofoil, it has a low pressure
area over the upper surface and high speed, and higher the
pressure at the lower surface. when the fluid flow through
an aerofoil. this creates a variance in temperature and
pressure inside the fuselage. So to minimize this effect
inside the fuselage, it is made of circular cross section.