1. I don't see why this is in RTS when it's clearly Tower Defense.
2. I don't see why the controls have to be so convoluted. (Click buy, click unit you want, click Unit, click the unit you want, click buy, place it, click to build or destroy. I mean...seriously?)
3. For being the first mission, it's insanely difficult. You only have one tower to defeat the Zergs with and you cannot do it placing it regularly. You have to move it around, which involves reacting rather quickly with said convoluted controls by disabling your tower, clicking it, moving it, clicking again, and clicking build, rather than a simple drag and release. Also it doesn't fire between the times you're doing all this, so better move fast.
Truly, why are such simple controls and mechanisms made to be so complex?