The game mechanics are simple. Not difficult. Fun.
What unit you choose in your current round directly effects how much money you have in the next. So choosing cheap units that give lots of income will eventually make you stronger, Regardless. Not being able to figure out what units your opponent will send out? Do they have lots of surviving units of a certain type? Is their Armour light? Send out the samurai bitches. Heavy? Reg swordsmen.
Then later on apprentices or ninjas for every other light wave.... Then once you have enough spare money (Like. 3 k surviving from the last wave.) Send out dread lords and win.