What are training slots and how do I get more of them?
A Training Slot refers to the ability to train a group of troops. For every Barrack you build, you will gain one additional Training Slot. The more Training Slots you have, the more groups of Troops you will be able to have in your training queue.
Increasing the amount of Barracks you have will give you more Training Slots and increase training speed. Upgrading the current Barracks you have will also increase training speed and unlock new troop types. Also, make sure you continue increasing and upgrading your Villas since the amount of troops you're able to train is based off your current Idle Population.