I would like to know if its normal that every time i bought a tower on this game , when i had to change of level all the towers i bought was gone with the blue(or green?) coins.
Yes it is normal, however there is a special level (called Monster Mishmash i believe) that is in the medium difficulty. It has you start off with a ton of money and you have all towers unlocked. Its a blast.
nice sig kasumakuma