Would much prefer wasd control (I'm new to construct - I presume that's possible?) so I didn't really play properly because of that. Also the rate of turret spawning seems too rapid - might want to make some delimiter number that rises slowly, so if the total number of turrets equals it it wont spawn any more turrets. Right now it banks on the player destroying all turrets to ensure a steady rate of challenge.
I see potential in it and the rampage style building collapses are neat!
Your in luck! It does have WASD control, I just didn't put it in the instructions. It also has the type of spawn process you described (spawns 1, then 2, and so on to 5 with a max of 15 being created at any one time). I could simply slow the spawn process to make it take longer to get overwhelmed, but being only one level, I didn't see the point. It's not a full game in that sense. It needs levels, powerups, different enemies (ground and air borne), more structures. With the free version all that is limited.