Ammunition

Each Tower, Fort and Keep had a limited amount of Ammunition (short Ammo), displayed beneath its remaining Health, with each attack consuming one charge. Once a structures stores had been exhausted, it was unable to attack, with a special icon displayed above it to indicate that it is out of ammo. However, structures did slowly replenish their ammo over time for around 2 charges every 30 seconds, at which point they resumed attacking any enemies within range.

Removed
Ammunition was removed with the on December 12, 2017 with 2018 Gameplay Update to increase the length and depth of the laning phase of matches. The Developers felt that it had begun to fall by the wayside for those that had figured out how to skirt the laning mechanics by just skipping from lane to lane, defeating enemy minions, and letting allied minions take down the towers by eating the shots until they ran out of Ammunition. It also makes a tower's impact in the game easy to understand: If it's up, it'll defend. If it's down, it won't. Ammo created an odd grey area.

Since ammo can no longer be depleted, they also reduced structure damage to minions, and increased minion damage to help them hit a little harder. These changes should give players a bit more incentive to attack with their minion waves in order to deal more meaningful damage to opposing towers.

One of the Nexus anomalies developers have tested internally revolves around bringing back Tower Ammunition, but even after speeding things up, it took too much time for Towers to reload, which felt generally bad for the game, so they decided to scrap it.