"The Zamorin's empire at its zenith included the whole of the west coast from Kollam to Kollam, that is from Panthalayini in Kurumbranad Taluk of British Malabar to Quilon in the Travancore State", writes Prof. K.V.Krishna Ayyar in his monumental work, 'The Zamorins of Calicut'.
Panthalayini Kollam was, of course, a well-known medieval port city, blessed as it is with natural mud banks which ensured calm water on the open coast all through the south-west monsoon.  'This is the Pandarani of Portuguese writers, the Flandarina of Friar Odoric, the Fandreeah of Rowlandson's Tahafat-ul-Mujahidin, the Fandaraina of Ibn Batuta', says Logan in his Malabar Manual. 
When news reached the Zamorin at Ponnani that Vasco da Gama's fleet was anchored off the coast of Kappad on 21st May 1498, his first instruction was to ensure that Gama should be escorted to Panthalayini Kollam, 'which was a good port unlike Calicut itself'. He was aware  that the south west monsoon with its devastating power was only ten days away and even the strongest fleet could be wiped off in its fury. (It was, however, a different story with the East India Company's vessel Morning Star which was totally wrecked in the fury of the south-west monsoon in 1793, while anchored at Panthalayini Kollam.)
Perhaps the folklore about the rivalry between Zamorin and Kolatthiri helped to confirm the belief that the boundary between the two principalities was the Korappuzha River.This may have contributed to the lack of any serious investigation of the reach of the Zamorin to the territory between Korappuzha and Panthalayani Kollam.
 How did the Zamorin come to acquire Panthalayini Kollam, about ten miles north of Korappuzha river? 
Krishna Ayyar quotes from Keralolpathi: The territory of Kolatthiri stretched from Korappuzha to Nileswaram. A prince from this family was stationed at Panthalayini Kollam as the southern viceroy. During one of his visits to Calicut, the young viceroy fell in love with a young princess (Thampuratti) of the Zamorin's family. They eloped to Kollam and from there to Chirakkal, the headquarters of Kolatthiri. Enraged at this, the Zamorin advanced against the Kolatthiri kingdom to take revenge against this insult to the family. He occupied Kollam and was marching towards Chirakkal.
 The Kolatthiri, however, sent emissaries offering to make amends for the wrong done by the viceroy. The Zamorin was pacified by offering Kollam and certain rights over the temple at Taliparamba. Thus it was that the Zamorin gained control over this prosperous port town.
Kollam became a favourite destination of the Zamorins till the Portuguese, the Dutch, the Mysore army and finally the British pinned them down to Calicut. The Calicut Granthavari records the demise of a Zamorin from Panthalayini Kollam in 1597 and the coronation of his successor  at the same location. But the records do not mention 'Panthalayini Kollam'. Instead, the name mentioned is 'Ananthapuram'. 
The Granthavari records an offering made by the Zamorin to the Ananthapurath Thevar in 1656 A.D. but no reference to the more famous Pisharikavu Temple which is next door. Perhaps, this Temple had an autonomous existence and the Zamorin confined himself to the Ananthapuram Temple. However, the Granthavari  does record the conferment of the title of Manappurath Arayan on a fisherman named Kodi son of Payyanad Kuttan in 1667. But the conferment of this title took place in the Calicut Palace, although the Manappurath Arayan had jurisdiction in Panthalayani Kollam.
Ananthapuram today is a small hamlet about half a kilometre to the north of Kollam town on the National Highway. It lies on the northern side of the vast Kollam Chira (tank) and more than a kilometre to the east of the port. Even the tank is not associated with the Zamorin - the local legend has it that a prosperous trader named Elela Chingan (some say Chingan Nair) dug the tank, which today is part of the Pisharikavu Temple complex.
The only vestige of the Zamorin in Ananthapuram today is a modest building which used to be the Palace of the Zamorin and a few Brahmin households which formed part of the agraharam. Then there is the small but majestic Maha Vishnu Temple which may have been built by either the Kolatthiris or the Zamorins - midway between the two more famous Ananthapuram Temples in Kasaragod and Trivandrum. 
Anathapuram Maha Vishnu Temple