IF I have to use something, it is the spikes. But my new stratigy involves little spikes. My key is not spending that money and letting the three to 10 baloons pass in doing so you can up your monkeys fairly fast. You need to spread them out to pop the metal baloons. If you don't, the metal baloons get popped too late and the fast yellows fly through and make you loose.and also I have found it more effective to buy 2 monkeys as apposed to upgrading the cannon. When you are able to fend the baloons off, you can start to upgrade. IMPO the med level is by far the easiest. Hard is super hard and I made it to level 30. I bought the cannon first put it to the left. Then I buy the monkey and put it toward the right. then bottom right, then bottom left. I thought I was going to beat it but I used too much money on spikes and glue. I really like this game