  We study the canonical quantization of a bosonic string in presence of N
twist fields. This generalizes the quantization of the twisted string in two
ways: the in and out states are not necessarily twisted and the number of twist
fields N can be bigger than 2.
  In order to quantize the theory we need to find the normal modes. Then we
need to define a product between two modes which is conserved. Because of this
we need to use the Klein-Gordon product and to separate the string coordinate
into the classical and the quantum part. The quantum part has different
boundary conditions than the original string coordinates but these boundary
conditions are precisely those which make the operator describing the equation
of motion self adjoint.
  The splitting of the string coordinates into a classical and quantum part
allows the formulation of an improved overlap principle. Using this approach we
then proceed in computing the generating function for the generic correlator
with L untwisted operators and N (excited) twist fields for branes at angles.
We recover as expected the results previously obtained using the path integral.
This construction explains why these correlators
