  We show how the low-energy properties of the 2-leg XXZ spin-1/2 ladders with
general anisotropy parameter $\Delta $ on closed geometries can be accounted
for in the framework of the m-reduction procedure developed in [1]. In the
limit of quasi-decoupled chains, a conformal field theory (CFT) with central
charge c=2 is derived and its ability to describe the model with different
boundary conditions is shown. Special emphasis is given to the Mobius boundary
conditions which generate a topological defect corresponding to non trivial
single-spinon excitations. Then, in the case of the 2-leg XXX ladders we
discuss in detail the role of various perturbations in determining the
renormalization group flow starting from the ultraviolet (UV) critical point
with c=2.
