There used to be small watchtowers in some parts of ancient Greece, e.g. (IIRC) near Nemea, apparently at the heads of valleys that were near the borders with adjacent states. Their small size would have implied fairly small garrisons, so it's not as if they could have stopped very much. Might they have been signal towers? What sort of garrisons might they have had? Hoplites? Psiloi?