Crystal Lands

Definitely needs better instructions. Let me give it a shot.
Each crystal can turn into two towers: one on the roof and one on the floor. Roof towers can be upgraded by clicking on them. You start with the green roof and purple floor towers.
Green roof is a single shot damage tower.
Green floor is a slow trap.
Purple roof is a splash damage tower.
Purple floor generates mana (and is absolutely necessary).
Red roof is a DOT/debuff tower.
Red floor is damage.
Blue roof is a slow tower.
Blue floor is damage.
All towers require mana to fire. So you need to balance purple mana stones with towers. Upgrades seem to be way more cost effective than tower spam. Towers can only shoot below themselves to start. Each time you fill up the yellow bar, you get an upgrade point. The abilities highlighted with a color unlock a new tower or trap. The mana/crystal ability and the ability for greens to shoot upwards are really powerful.

If you open the level-up screen, on the bottom right there is a book icon. Clicking it brings up an explanation of what each tower type does.

To back up my earlier lightning + mana advice, just did a quick challenge run (only purple crystals, no damage taken, no superweapon) through all 9 levels. While you'll need to have maxed mana and gold upgrades for the last levels, it is possible to finish the game with a single colour.

why is there still flash games where it's impossible to zoom with the browser ?

This needs better instructions. The on screen explanations of what each crystal does are not clear enough. This game isn't good enough that I'm willing to lose over and over again just to learn how to play it.