If you are from abroad, the nearest airport is New Delhi airport. From here you can take one of the several connecting Duranto Express trains from Delhi to Allahabad including Kolkata Rajdhani express, Prayag Raj Express, and Duranto Express. The Allahabad airport is also called Bamrauli Field. However, this is essentially a military base which receives one commercial flight run of Air India per day from Delhi to Allahabad via Kanpur. It is far easier to fly in to Varanasi or Lucknow and then take a bus or train to the city.