Etymology tree Proto-Indo-European *h₂el- Latin ūls Proto-Indo-European *-teros? Latin -ter Old Latin -ād Latin -ā Latin ultrā From ūls + -ter + -ā (adverb ending). See also citrā, intrā, extrā. The accusative is from the pre-PIE directional.
Practice speaking and memorizing "ultra" and many other words and sentences in Latin.