Here’s my Tower Defense game. Added bunch of extra features such as leveling, multiple paths, mana etc.
I feel like I found a good balance in game. One game last around 2 to 3 minutes before it gets too hard. Or maybe I’m just bad player 
Full list of rules and features:
Placing or moving a tower cost 5 mana.
You gain 1 mana in every 3 seconds.
Tower upgrades themselves.
Upgrading costs 3 mana and it is automatically taken away from your mana pool.
If you don’t have enough mana you lose one of your towers.
Max number of towers is 5.
Number on top of the tower (for example 1/34%) indicates tower LVL 1 and progress towards to next level 34%.
Towers gain little experience for hitting enemy and much more when killing enemy.
Towers become stronger when they level up.
When towers level up their experience requirement also increases so leveling will be slightly slower for high level towers.
Enemies come in waves.
Game starts at wave 1 and wave is increased in every 5 enemies.
Every wave increases enemy’s health, speed and damage it makes to player’s base.
You can place towers basically anywhere. If you place it on top of current path all new enemies will find new way (existing enemies go through towers if they aren’t dead by then).
If you place towers so that no path can be created (aka. blocking enemies) you lose all your towers you cheater 
That’s all I think. Here’s a video me playing one game
Play here (press to fullscreen to see whole UI): https://www.sharemygame.com/share/b91b3fe4-4d3b-46cb-ac55-9b374122c2ca