In this contribution we study the ground states and the low-energy excitations of a wide class of antiferromagnetic two-leg spin-1/2 XXZ Heisemberg ladders, arranged in a closed geometry, for different boundary conditions, which range from periodic to Mobius ones. We employ a twisted conformal field theory (CFT) approach [1], already successfully applied to quantum Hall systems in the presence of impurities or defects [2], to Josephson junction ladders and arrays of non trivial geometry [3] and to the study of the phase diagram of the fully frustrated XY model [4].\
As a result the CFT description captures its universal features and reproduces the basic phenomenology [5]. In particular we focus on the weak coupling limit between the two legs of the ladder, well described by a CFT with central charge c = 2, and discuss in detail the isotropic XXX ladder and the strongly anisotropic XX ladder cases. Finally, specializing to the XXX case we analyse in detail the role of various perturbations and the resulting renormalization group flow from the ultraviolet (UV) critical point with c = 2 to several infrared (IR) fixed points with a lower central charge [5]. \
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[1] G. Cristofano, G. Maiella, V. Marotta, G. Niccoli, Nucl. Phys. B 641 (2002) 547; G. Cristofano, V. Marotta, G. Niccoli, JHEP 06 (2004) 056.\
[2] G. Cristofano, V. Marotta, A. Naddeo, Phys. Lett. B 571 (2003) 250; G. Cristofano, V. Marotta, A. Naddeo, Nucl. Phys. B 679 (2004) 621; G. Cristofano, V. Marotta, A. Naddeo, G. Niccoli, J. Stat. Mech.\: Theor. Exper. (2006) L05002.\
[3] G. Cristofano, V. Marotta, A. Naddeo, J. Stat. Mech.\: Theor. Exper. (2005) P03006; G. Cristofano, V. Marotta, A. Naddeo, G. Niccoli, Phys. Lett. A 372 (2008) 2464.\
[4] G. Cristofano, V. Marotta, P. Minnhagen, A. Naddeo, G. Niccoli, J. Stat. Mech.\: Theor. Exp. 11 (2006) P11009.\
[5] G. Cristofano, V. Marotta, A. Naddeo, G. Niccoli, J. Stat. Mech.\: Theor. Exp. (2008) submitted.\