Analytic expressions for the magnetic field of a toroidal stellarator produced by a set of finite-size helical conductors wound on a torus have been obtained by a direct application of the Biot-Savart law. The effects of toroidal curvature have been included as toroidal perturbations which break the helical symmetry of a corresponding straight system. On expressing the problem of field line behaviour as the problem of a conservative Hamiltonian dynamical system under small non-stationary perturbations, approximate toroidal magnetic surfaces have been obtained by an asymptotic method. Numerical examples for simple cases are given to illustrate the effects of toroidal curvature.