"Palenque's later rulers continued to enlarge their splendid residence, the Great Palace, in particular raising the famed and unique four-story tower. Although evidence for its construction date is yet to be found, Linda Schele has hypothesized that it was built by Kan Xul II in veneration of his father, Pacal.
When viewed from the top of the Great Palace tower on the winter solstice, the sun sets directly behind the Temple of the Inscriptions, appearing to enter the underworld throught Pacal's tomb."
Robert Sharer, The Ancient Maya, p. 293-4
From the Maudslay Collection, British Museum. Used with permission under the CC BY-NC-SA 4.0 non-commercial license. ©e Trustees of the British Museum.
"Kan Xul II also began the construction of the northern building of the Palace. Excavations conducted by Albert Ruz Lhuillier there uncovered a stone panel carved with extensive hieroglyphic text and an accession scene.
The text reviews the preceding reigns and appears to depict Kan Xul at his inauguration, attended by his mother. But it also contains an ominous clue to a disaster that befell Palenque's ruling house."
Robert Sharer, The Ancient Maya, p. 293-4
"Although no record of the event has been found at Palenque, Kan Xul II apparently led a raid on the neighboring site of Tonina [75 miles to the south over a mountain range] in order to secure the captive sacrifices necessary to the dedication of his new palace. But fate dictated otherwise; at Tonina archaeologists have discovered a record of Kan Xul's capture by Ruler 3 of Tonina.
That Kan Xul II had been taken captive and was apparently held for a long period before eventually being sacrificed must have plunged the Palenque polity into despair, for the tablet at Palenque's Great Palace records that the new building was completed and dedicated by a man named Xoc, not as ruler but apparently as a stand-in during this interregnum.
It would appear that a new king could not assume the throne while the old king still lived, notwithstanding that the royal captive at Tonina could no longer exercise rule."
Robert Sharer, The Ancient Maya, p. 293-4
In addition to its ceremonial purpose, the tower probably served a defensive purpose as well, as the northwesterly view from the top surveys the broad coastal plain stretching away to the Gulf of Mexico.