While the base game is usually a one-time purchase on mobile and PC, the original Kingdom Rush is the only entry that is consistently free-to-play on mobile devices. Frontiers has historically been offered for free during events like the 2020 lockdowns, so it is worth checking the stores during major holidays or Ironhide anniversaries.
: Set towers in diverse environments like deserts, jungles, and caves. Kingdom Rush: Frontiers Free Download
: 8 specialized tower upgrades with over 18 unique abilities, including Death Riders and Earthquake Machines. While the base game is usually a one-time