Ok so the concept has potential, but the game is lacking. Also calling this a tower defense is a little misleading.
The objective of the game is a little confusing: "...instead of killing enemies, you monitor their movements. The goal is to stop 100 crimes."
So I'm trying to stop crimes, and once I stop 100 I win. That's all well and good, a clear objective, but what crimes am I stopping?
How are your representing a crime?
-Some of the people seem to be colored yellow, are they the criminals?
-Are they always yellow, or only after committing a crime?
-Do they commit multiple crimes?
-What crimes are being committed? all I see are dots moving around, are their murders taking place, theft, burglaries?
-Are criminals breaking into businesses?
I see the pedestrians moving, mostly on sidewalks (thats good) but they don't seem to react to the siren if they are in a crosswalk. Maybe getting them to move out of the crosswalk and roads if they are within a 1.5 block radius of the cruiser.
Try making a toggle button in a corner for muting/unmuting, this is or in a pause menu is really the best way to mute/unmute.
What do the cameras do? What is the point of having them? Do cameras deter criminals?
The in game economy seems a little out of whack, I was easily able to place a camera at every intersection, if they can see 1.5 blocks then I had the entire map covered without running out of money. Maybe try adding other forms of surveillance: wire taps, private security, surveillance units that would set up in a building to watch the street outside/building across the street. Each of these could have a cost. Try tying the players ability to prevent crime to their budget, if the player prevents crime they get more money, if the criminals get away, then they dont get money. Which actually seems a little backwards, maybe reverse that, the more criminals escaping, the more money in your budget. If your stopping all the crime, why would the city keep throwing money at you.
I would ditch the control of one cruiser and add multiple officers/cruisers that would each have a price tag (every cruiser costs $x per sec, every patrol officer costs $y per sec) you could control them by dragging boxes around them to select units and clicking an area on the map to direct their attention to said area.
Anyways those are just some of the ideas that came to mind during my play-through.
Good luck on your future projects!