  A twisted graphene bilayer exhibits a triangular Moir\'e pattern in the local
stacking, that smoothly alternates between the three basic types AA', AB' and
BA'. Under an interlayer bias U, the latter two types develop a spectral gap,
characterised by opposite valley Chern numbers. We show that for large enough
Moir\'e periods and bias, these regions become depleted electronically, and
topologically protected helical modes appear at their boundaries. This gives
rise to a delocalised topological network of the Chalker-Coddington type,
composed of valley current vortices. This network can be tailored by controlled
deposition of valley-mixing adsorbates, which block transmission in selected
links, thus opening the possibility of custom topological nanoelectronics.
