We present the results of the monitoring of solar lighting inside cave n. 3 of the aboriginal archaeological site of Cuevas del Barranco de la Angostura (Agüimes, Gran Canaria). The cave has a cylindrical shape with a vaulted roof and topped by a skylight, similar to other caves where the existence of astronomical markers has been proposed. We find that the Sun illuminates the interior for only half the year, from about ten to eleven days before the autumnal equinox to ten to eleven days after the spring equinox. The most convincing result is the discovery of a striking marker of the winter solstice, since the first rays of the sun that illuminate the interior on that date do so just inside a small painted artificial hole located on the north wall of the cave. We also found a possible marker of the equinox or the half-day between solstices based on the behavior of the solar illumination pattern on those dates.