intus

Intus is an adverb and only an adverb. It answers "where?", and in older Latin it can also answer "from where?" - intus venire is to come from within. What it cannot do is take an object. For "inside the walls" you need intra with the accusative, intra muros, or in with the ablative; intus muros is not Latin, however tempting the English word order makes it.

Its natural partner is foris, "outside", and the pair are stock furniture in moral writing about what a person is on the inside as against what they show. Note the -us ending is adverbial here, the same formation as penitus and funditus, not a second declension nominative. English inherits interior, internal, intestine and intimate from the same in- family, while intus itself survives only in medical and legal tags.