  We show that every self-orthogonal code over $\mathbb F_q$ of length $n$ can
be extended to a self-dual code, if there exists self-dual codes of length $n$.
Using a family of Galois towers of algebraic function fields we show that over
any nonprime field $\mathbb F_q$, with $q\geq 64$, except possibly $q=125$,
there are self-dual codes better than the asymptotic Gilbert--Varshamov bound.
