  We study the global structure of vacua of heterotic strings compactified on
orbifolds $T^4/Z_N$ (N=2,3) in the presence of heterotic 5-branes. Gauge
symmetry breaking associated with orbifold is described by instantons in the
field theory. Phase transition between small instantons and heterotic 5-branes
provides top-down, stringy account to the spectrum and modular invariance
condition. Also it takes us from one vacuum to another by emitting and
absorbing instantons. This means that many vacua with different gauge theory
are in fact connected and are inherited from perturbative vacua. It follows
that there are also transitions among twisted fields, heterotic 5-branes and
instantons.
