There was . . . . . . . . big notice board outside . . . . . . . . town hall.

In the given sentence, the indefinite article, 'a' before the noun phrase, 'big notice board' is used to mean 'a certain notice board' or 'one notice board'.
The word, 'town hall' is a noun used to refer to a building used for the administration of local government. In any city, there is usually only one town hall. As it refers to a particular noun (i.e the town hall of the particular city), the definite article, 'the' is used before the word, 'town hall'.