The campaign needs some balancing, you can only afford 1 tower at the beginning, and even placing it where it has the most coverage it can't beat the first wave. It took 2 waves to get a second tower, and it was just  a duplicate of the first. Would have like to see the ability to earn more towers faster/sooner to see how they could combine. Your towers also didn't prioritize the first target in range, which meant that sometimes it would let an enemy pass when it could have killed it.