Not bad!
Here are some preliminary comments... Mostly quick notes I wrote while playing the first 15 minutes...
The game is nice and has a good feel to it
You should add special icons to clearly show the player what are the units that attack and those who harvest. Since you already divided you units in groups, all you need to do is to make icons to show those groups to the player
I would make it clearer that when you cannot build a tower. I know you have a red outline in the unit bar but the warning should be focused in-game. Make a big red X appear ove rthe "ghost" building or make the ghost red or have a message written over it saying "cannot build"
Whenever you make a move that results negatively, like trying to build something when you don't have the money or placing somewhere it doesn't fit or anything else you can think of, play a sound effect and write it on screen.
When I select a unit I should be able to deselect it by right clicking since you're already using that control when building. Left-click selects, right-click deselects.
At first, I thought that the provider of energy would shoot their power to the nearest tower in need. For example, if you put focus on battle, the energy shooters (dunno their names yet) would concnetrate on feeding their energy to a tower that shoots something while in battle, which mean that harvesters would have less energy hence harvest less.
Example:
Let's say your feeder shoot 5 energy per sec
Your attacker need 10 energy to shoot normaly, 20 to be at full power.
Your harvester needs 5 energy to harvest normaly, 10 to be at full power
Below these energy, they attack/harvest at half speed. If no energy they become idle
Taking that you have a setup where a feeder is placed between an attacker and an harvester:
Using the attack priority, the feeder will shoot his energy to the attacker only when under an attack, putting the harvester idle and the tower at full power
Using a harvester tactic, the feeder will feed 3 energy to the attacker and 2 to the harvester, putting the attacker at low rate but still defends and the harvester would still be able to harvest at low rate
All in all, the player would have to manage where he places his feeders, attackers and harvesters while also switching form one priority to another. Anyway, it,s just a thought, perhaps the system is very good as it is, but my first reaction to the game was that I didn't understand why the feeder wouldn't give energy to the tower I was trying to build...
Speaking of which, I had a hard time reacting to an attack. An alien attacked me from the rear and i was trying to build a new attacking tower but couldn't because the feeder would still feed the harvesters. Almost cost me the game because of that silly saucer... Well, in a way it did cuz I started a new game
What's the un-link supposed to do when selecting a tower?
Are you planning of having money bonuses for destroying alien ship? Like selling scrap metal or using it to build special units?
I would give a clear warning when enemies are aproaching. Perhaps add a radar functions on the main buildings that react when an enemy is closing in...
Perhaps it's in relation with the last point, but it felt like ages before I got to kill something. I even checked around the map to see if there were someone standing in a corner or something...
Speaking of which, our goal isn't very clear at first. Perhaps you could have an alien base arrive to tell us that there will be an enemy at some point of the game. Anything to tell us why to build. At my very first try, I figured no one would attack so I only made harvesters and feeders and only one attacker. Could be interesting to have enemy foot soldier moving toward our base.
Do you have plan of adding units to the player?
Enemies should have clearer attack. Perhaps you could select specific colors for the player attacks and enemy attacks so that whenever we see a color we know who is attacking who. For example, player shots could be blue and enemy shots red (classic scheme), but it could also be green and purple or black and white... Anything you can imagine as a matter a fact...
Perhaps a timer written somewhere could be nice to tell us how much time we've been playing but I'm not totally sure about the idea.
Is the threat level associated to us or to the enemy? If you want to tell us how much the enmy is threatning us, perhaps you should place the info closer to the mini-map instead of where you tell us our personal info such as how many cash we have.
Anyway, like I said, I'll try to play a bit more cuz I only tried it out for 15 minutes or so...
Good game in the way!
Later!