The Czech word "věž" (tower) has a feminine gender. In Czech (as in Slovak), the suffix -ka also has a feminine gender, so in this case they took the root word (Dalibor) and added a feminine suffix since it was referring to a tower. This is a practice common in both Czech and Slovak.

A comparable case is the Slovak village of Turzovka.

edit: I am not a historian, just a speaker of Czech and Slovak.