The ladder (or staircase) is symbolic of the connection between HEAVEN and EARTH. It represents progress, ascension, and spiritual passage through the levels of initiation.
In the Bible, Jacob's ladder established contact between man and God, and there are seven rungs on the ladder of virtue (Gen. 28:10-17). Generally a ladder is necessary to reach places otherwise inaccessible. A ladder is not merely an aid to reach those places; it is an essential piece of equipment.
It can also connote ambition, as in the gradual acquisition of knowledge, as well as overcoming a problem.