In the dam level, there are three guard towers on the main dam, and one across the lake on the hidden island. The ones on the dam are marked 2, 3, and 4 in order, and the one on the island doesn't have a number. This leaves guard tower 1 missing from the game.

It's possible that this tower originally appeared immediately to the right of the locked gate to the dam. When it was eventually removed, the developers may have simply forgotten to change the other numbers.