Some strategy advice. Create 2 kill zones - one clustered around the mothership (do this first) and one around the sheep pen. iron tower upgrades are useful early on. I like to use the multi-shot ones near the mothership - lots of targets so they don't miss. Then make some flame towers. The critical element is the use of freeze towers - they will lock the UFOs in the kill zones. Set up a line of freeze towers so that any UFO will be stopped and killed. Try pairing each freeze tower with a flame tower. As you acquire significant money build high-level missile towers. Good Luck!
need ability to pause and build at start of a level - you can't spend initial money in a useful way. I only lose lives on the first few waves as a result of this.