intra

Intra takes the accusative, always, with no exceptions worth the ink: intra muros, intra annum. It works in space, meaning enclosed by something, and in time, meaning inside a stretch of it rather than after the stretch has ended.

Its opposite is extra, which also governs the accusative, and the two sit either side of inter. That last pair is the trap: intra muros is inside the walls, inter muros is between two of them, and one letter separates a defended city from a gap. As a prefix intra- gives intramural and intravenous. The intro- of introduco is a different word, the adverb intro, "inwards", which is why you go intro but stay intra.