Clear as many boxes as possible and build up a large force before trying to conquer your first city!
Control your bubble with your mouse, click to clear the boxes when they are inside the box. More boxes means more coins! Don't touch the spikes and collect the coins to buy upgrades between levels.
Tips & Tricks
The first one in is the first one dead. If you are low on health potions don't risk being in the middle of a swarm of enemies, stay to the outside and take them out one at a time.
Enemies drop magical weapons. Some of these can be far better than the ones you get in the merchant shop.