The aim of Flash Element Tower Defense is to kill all the creeps before they reach the end of the maze. Like most tower defense games, you build towers that have different attributes. In FETD, there are two different sets of towers: basic towers, and elemental towers.
The three basic towers are Arrow, Cannon, and Air towers. The arrow tower is a good all-around tower, as it is weak but fast, and can hit both land and air creeps. The cannon tower is strong and slow, has splash, and only hits land creeps. The air tower is fast and strong, but only hit air creeps.
The three elemental towers are Water, Earth, and Fire towers. Elemental towers are more expensive than basic towers, and to build them, one must use wood which is gained after every seven levels. Each tower has a different element to research. The wood can also be used to increase interest. The Water tower does medium damage but fires fast, it slows creeps, and hits land and air creeps. The Earth tower is like the cannon tower, but does more damage and has longer range. The Fire tower does large damage, fires slowly, has splash, and hits both air and land creeps.
The last tower is the combo tower. It requires that you have researched all three elements. It costs 200 gold, but has huge damage and range, and can be upgraded to be twice as powerful damage wise.
A known problem about the game is that you can overlap towers by being patient and careful of how to place them down. Tutorials for these can be found on Youtube, or you could just experiment yourself.
The special creeps are fast (6, 13, 19, 26, 32), air (8, 17, 27), immune (10, 21, 32), and boss (11, 22, 33).