Enemy placement makes for challenging action, but there's absolutely no reason a Koopa should slide into you within the first second of the start of level 2. On the other hand, what comes after that is a test of patience and skill with the Amazin' Flyin' Hammer Bro vs. you on a skinny pipe. I found that to be the most memorable instance of strategic enemy usage; that guy hounded the heck out of me. Getting the secret exit is pretty hard, as you only have one shot at the key. The first level offers a little bonus area up in the sky as a break from all the action where you have a limited time to collect coins and only if you hit the switch in the right area.