  We introduce the concept of multisymplectic formalism, familiar in covariant
field theory, for the study of integrable defects in 1+1 classical field
theory. The main idea is the coexistence of two Poisson brackets, one for each
spacetime coordinate. The Poisson bracket corresponding to the time coordinate
is the usual one describing the time evolution of the system. Taking the
nonlinear Schr\"odinger (NLS) equation as an example, we introduce the new
bracket associated to the space coordinate. We show that, in the absence of any
defect, the two brackets yield completely equivalent Hamiltonian descriptions
of the model. However, in the presence of a defect described by a frozen
B\"acklund transformation, the advantage of using the new bracket becomes
evident. It allows us to reinterpret the defect conditions as canonical
transformations. As a consequence, we are also able to implement the method of
the classical r matrix and to prove Liouville integrability of the system with
such a defect. The use of the new Poisson bracket completely bypasses all the
known problems associated with the presence of a defect in the discussion of
Liouville integrability. A by-product of the approach is the reinterpretation
of the defect Lagrangian used in the Lagrangian description of integrable
defects as the generating function of the canonical transformation representing
the defect conditions.
