Sentry Gun is the first upgrade of path 1 for the Monkey Engineer in Bloons TD 5 Deluxe, Bloons TD 5 Mobile, Bloons Monkey City and Bloons TD Battles Mobile that allows it to create sentry guns within its range. The Sentry Guns shoot nails, and each nail can pop 2 bloons. It is very useful for the early rounds as it acts like several Dart Monkeys all attacking at once. The Monkey Engineer can create up to 3 Sentry Guns with no fire rate upgrades when he is upgraded to this. However, the Engineer can produce more sentries if it's under the influence of a Jungle Drums Monkey Village, M.I.B Call to Arms ability, or if the Fast Engineering and/or Sprockets upgrades are bought.