Kāśī is another name for Vārāṇasī (Benares). It has been a place of pilgrimage since time immemorial. Two rivers named Asiḥ and Varuṇā merge there. Maṇikarṇikā is famous because, according to the opinion of great personalities, a bejeweled earring fell there from the ear of Lord Viṣṇu. According to some, it fell from the ear of Lord Śiva. The word maṇi means "jewel," and karṇikā means "from the ear." According to some, Lord Viśvanātha is the great physician who cures the disease of material existence by delivering a person through the ear, which receives the vibration of the holy name of Lord Rāma. Because of this, this holy place is called Maṇikarṇikā. It is said that there is no better place than where the river Ganges flows, and the bathing ghat known as Maṇikarṇikā is especially sanctified because it is very dear to Lord Viśvanātha. In the Kāśī-khaṇḍa it is said:
saṁsāri-cintāmaṇir atra yasmāt
tārakaṁ saj-jana-karṇikāyām
śivo ’bhidhatte saha-sānta-kāle
tad gīyate ’sau maṇi-karṇiketi
mukti-lakṣmī mahā-pīṭha-maṇis tac-caraṇābjayoḥ
karṇikeyaṁ tataḥ prāhur yāṁ janā maṇi-karṇikām
According to this passage from the Kāśī-khaṇḍa, one who gives up his body at Maṇikarṇikā is liberated simply by remembering Lord Śiva's name.
(Sri Caitanya Caritamrta-------2:17:82------purport).

When Lord Caitanya was in Benares promulgating the chanting of Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare, thousands of people were following Him. Prakāśānanda Sarasvatī, a very influential and learned scholar in Benares at that time, derided Lord Caitanya for being a sentimentalist. Sometimes Māyāvādī philosophers criticize the devotees because they think that most of the devotees are in the darkness of ignorance and are philosophically naive sentimentalists. Actually that is not the fact. There are very, very learned scholars who have put forward the philosophy of devotion. But even if a devotee does not take advantage of their literatures or of his spiritual master, if he is sincere in his devotional service he is helped by Kṛṣṇa Himself within his heart. So the sincere devotee engaged in Kṛṣṇa consciousness cannot be without knowledge. The only qualification is that one carry out devotional service in full Kṛṣṇa consciousness.
(Bhagavad-Gita--------10:11-------purport).

On His way back from Vṛndāvana He first came to Prayāga, where He met Rūpa Gosvāmī along with his younger brother, Anupama. Then He came down to Vārāṇasī(Benares), where he became the guest of Śrī Tapana Miśra and Candraśekhara, assisted by a Mahārāṣṭra brāhmaṇa. At that time Vārāṇasī was headed by a great sannyāsī of the Māyāvāda school named Śrīpāda Prakāśānanda Sarasvatī. When the Lord was at Vārāṇasī, the people in general became more attracted to Lord Caitanya Mahāprabhu on account of His mass saṅkīrtana movement. Wherever He visited, especially the Viśvanātha temple, thousands of pilgrims would follow Him. Some were attracted by His bodily features, and others were attracted by His melodious songs glorifying the Lord.
(Srimad Bhagavatam---------introduction).

After thus delivering the people in general, the Lord desired to leave Vārāṇasī. After instructing Śrī Sanātana Gosvāmī, He sent him toward Vṛndāvana.
The actual purpose of Lord Caitanya's stay at Vārāṇasī after coming back from Vṛndāvana was to meet Sanātana Gosvāmī and teach him. Sanātana Gosvāmī met Śrī Caitanya Mahāprabhu after the Lord's return to Vārāṇasī, where the Lord taught him for two months about the intricacies of Vaiṣṇava philosophy and Vaiṣṇava activities. After completely instructing him, He sent him to Vṛndāvana to execute His orders. When Sanātana Gosvāmī went to Vṛndāvana, there were no temples. The city was lying vacant like an open field. Sanātana Gosvāmī sat down on the bank of the Yamunā, and after some time he gradually constructed the first temple; then other temples were constructed, and now the city is full of temples, numbering about five thousand.
(Sri Caitanya Caritamrta-------1:7:160--------translation and purport).

In the Caitanya-bhāgavata (Antya-khaṇḍa, Chapter Two) it is said that when Lord Śrī Caitanya Mahāprabhu arrived at Śrī Bhuvaneśvara, He visited the temple of Lord Śiva known as Gupta-kāśī (the concealed Vārāṇasī). Lord Śiva established this as a place of pilgrimage by bringing water from all holy places and creating the lake known as Bindu-sarovara. Śrī Caitanya Mahāprabhu took His bath in this lake, feeling a great regard for Lord Śiva. From the spiritual point of view, people still go to take a bath in this lake. Actually, by taking a bath there, one becomes very healthy even from the material viewpoint. Taking a bath and drinking the water of this lake can cure any disease of the stomach. Regular bathing certainly cures indigestion. The river Bhārgī, or Bhārgīnadī, came to be known as the Daṇḍa-bhāṅgā-nadī after Lord Caitanya bathed in its waters. It is situated six miles north of Jagannātha Purī. The reason for the change in names is given as follows.
(Sri Caitanya Caritamrta-------2:5:141------purport).

While at Vārāṇasī (Benares), Śrī Caitanya Mahāprabhu stayed at the house of Tapana Miśra. Near Tapana Miśra's house was a bathing ghat known as Pañcanadī-ghāṭa. Śrī Caitanya Mahāprabhu used to take His bath daily at this ghat, and He used to see the temple of Bindu Mādhava. Then He would take His lunch at Tapana Miśra's house. Near the Bindu Mādhava temple is a big banyan tree, and it is said that after eating, Śrī Caitanya Mahāprabhu used to rest beneath the tree. That banyan tree is still known today as Caitanya-vaṭa. Gradually, due to changes in language, the name became Yatana-vaṭa. The local people still call that place Yatana-vaṭa.
(Sri Caitanya Caritamrta-------2:17:89------purport).

The word prārabdhe ("past deeds") is important in this verse. Since Candraśekhara was a devotee, he was always eager to hear about Kṛṣṇa and His transcendental pastimes. Most of the inhabitants of Benares were and are impersonalists, worshipers of Lord Śiva and followers of the pañcopāsanā method. The impersonalists imagine some form of the impersonal Brahman, and to facilitate meditation they concentrate upon the forms of Viṣṇu, Śiva, Gaṇeśa, Sūrya and goddess Durgā. Actually these pañcopāsakas are not devotees of anyone. As it is said, to be a servant of everyone is to be a servant of no one. Vārāṇasī, or Kāśī, is the chief holy place of pilgrimage for impersonalists, and it is not at all suitable for devotees. A Vaiṣṇava likes to live in a viṣṇu-tīrtha, a place where Lord Viṣṇu's temples are present. In Vārāṇasī there are many hundreds and thousands of Lord Śiva's temples, or pañcopāsaka temples. Consequently Candraśekhara expressed great unhappiness as he informed Lord Caitanya that he was obliged to live at Benares due to his past misdeeds. As said in the Bhakti-rasāmṛta-sindhu, durjāty-ārambhakaṁ pāpaṁ yat syāt prārabdham eva tat: “According to one's past misdeeds, one takes birth on a lower platform.” But in the Brahma-saṁhitā (5.54) it is said, karmāṇi nirdahati kintu ca bhakti-bhājām: "There is no karma attached to the past deeds or misdeeds of one in devotional service." A devotee is not subjected to karma-phala, the effect of fruitive activity. Karma-phala is applicable to karmīs, not bhaktas.
(Sri Caitanya Caritamrta-------2:17:95------purport).