Guruvayur's importance is out of proportion to its size, because the campaign fought there in the 1930s changed the law of temple entry in India, and its limits are visible in the rule that still governs the door.
The satyagraha
In the early 1930s the Guruvayur Satyagraha, led by K. Kelappan with A. K. Gopalan among the volunteers, demanded entry for the communities then classed as untouchable, who were excluded not only from the temple but from the roads around it.
A volunteer was beaten severely enough to make the campaign a national cause. Gandhi supported it and corresponded about it at length. Kelappan fasted.
The proclamation
The Temple Entry Proclamation of Travancore, issued on 12 November 1936, opened that state's temples to all Hindus regardless of caste. It was the first such measure anywhere in India, and Gandhi called it a modern miracle.
It did not cover Guruvayur, which lay in Malabar rather than Travancore, under direct British administration. Guruvayur was opened to all Hindus only on 2 June 1947, eleven years later and weeks before independence.
What the law did and did not do
The Constitution of India, and the temple-entry legislation that followed, abolished exclusion by caste from Hindu religious institutions. That was a genuine and hard-won change, and Guruvayur is one of the places it was won.
It left exclusion by religion exactly where it found it.
That is the line still drawn at Guruvayur's gate: any Hindu may enter regardless of caste, and no non-Hindu may enter at all. There is no declaration procedure of the kind Tirupati offers.
The bar rests on custom rather than on any enabling statute, and its legal basis has been questioned on precisely that ground, a restriction with no positive legal instrument behind it, operating inside a board that is itself a creature of statute and supervised by the Kerala government.
Cases have arisen involving people of Indian descent, people married into Hindu families, and public figures whose religion was disputed at the gate. They have not changed the rule.
The dress code
Also enforced, and also the board's: men uncover the upper body and wear a mundu; women wear a sari or equivalent. It is applied to everyone, and it is the restriction visitors most often encounter first.