I started playing this game and it seemed like a fun tower defense strategy game. The first few levels were fun to play and then replay in the restricted night mode, trying to figure out tactics for perfect scores. Later levels began to get harder, but not from a tactics/strategy standpoint. First, it introduces enemies that use cheap invisibility tricks to bypass towers combined with immunity to your stopgap spells. They aren't hard to defeat with proper planning, but often get through if you lose track of them. Then there are levels with multiple paths that need defending, but if the computer sends an entire wave down one path early on, you get overrun. The final few levels put you in a position where the ONLY way to win is to get lucky drops early in the level. If a certain obstacle isn't cleared by a certain round, it begins to cost more to defend each round than you can possibly earn, which is fine IF it's possible to get there without luck. I much prefer a game that can be won with proper strategy regardless of whether the RNG god is on your side. Finally, the final boss arrives and turns the game into a "click the smoke bombs as fast as you can" game. No more strategy, just knock the bombs away or you lose. It would be like playing a turn based RPG through to the end, and having to win a rhythm battle against the end boss.
This is an old game, but I'll offer a few ideas on bugs/features anyway:
Picking up drops is messed up. Potions and gold you have to click directly on, but trap scrolls are activated if any part of the cursor is touching them. This can make it impossible to collect needed resources without wasting valuable traps. Make the interface more consistent and show which item will be collected in the status bar.
Also, enemy corpses and graphic effects can prevent clicking on them at times, which is very annoying if you need to activate a trap in a short time window. Make sure the sparkly effects don't interfere with functionality.
The game eats up more and more resources until it eventually chokes the computer.
Showing the range would help when selecting tower upgrades.
Some kind of indicator is needed for the gems, for the times you are going for the perfect score. Often, it's hard to tell if an enemy picked a gem up and died, or died right before. This can be especially difficult if you have the skill where gems move back on their own. Perhaps a "Number of Gems Left" indicator that changes color once they've been disturbed. It's very annoying to play a level for 30 minutes and then find out that you lost the bonus.