What are the structures on the inside (lake side) walls of arch dams that look like medieval towers?

Hoover Dam has two pairs of them, on the left and the right side of the inside, lake side walls of the dam. They're joined together by concrete "roads" or "bridges" connecting them. Do they have something to do with measuring the level of the lake? Or are they drainage structures that take the place of spillways and channel the water down deep into the dam to the hydroelectric power plant? Or what are they for?