  We introduce - as a generalization of cyclic codes - the notion of transitive
codes, and we show that the class of transitive codes is asymptotically good.
Even more, transitive codes attain the Tsfasman-Vladut-Zink bound over F_q, for
all aquares q=l^2. We also show that self-orthogonal and self-dual codes attain
the Tsfasman-Vladut-Zink bound, thus improving previous results about self-dual
codes attaining the Gilbert-Varshamov bound. The main tool is a new
asymptotically optimal tower (E_n) of function fields over F_q where all
extensions E_n/E_0 are Galois.
