Hello Folks,
while playing the game i was looking for the easiest and most effective way to spend my barrels while playing through Easy / Normal / Hard one after another. The following information is of course just my personal opinion and might not /actually/ be the most efficient way in terms of numbers, but works great nontheless!

Before you upgrade towers or anything else you should really increase your income first! Buy the first upgrade for starting money + generated money on the third tab:


After that you should buy all the three first upgrade for your Ballista.

They only cost 4 Barrels each - which is a cheap bonus.
Then you upgrade the damage to maximum, followed by Range / Cost reduction one level each.

Example:
Start: 1 - 1 - 1 Star
Maximize Damge: 6 - 1 - 1 Star
Equally upgrade the rest: 6 - 2 - 2 Stars -> 6 - 3 - 3 Stars [...]

Once you maximized the ballista, it's time to choose a spell for your Achievement (Fully upgrade a spell). I would suggest the Posion Spell, as he seems to be the most effective against lots of enemies!


The order is up to you, but i first completely upgraded the damage here as well.

The most important thing is to quickly build a lot of towers. Due to the normally quite low damage you require more towers for the bigger ships.

Don't forget to leave a few spots (only after level 10+) for booster-mines so you can improve your towers later on.

Once you have built towers on all available spots, you can set up your mines. Choose 1 tower in a good spot and build him up to his maximum tier. He should be in range of the mine.
After that you can max. all your mines to boost your towers! Lvl 2 of your Balistas is not yet needed. Only after you maximized your mines you will want to build up all other towers to lvl 2.

Lvl 3 + 4 should be choosen carefully. I usually do not spend resources on getting all towers to lvl 3, but first build up 2-3 chosen Balistas and push them to lvl 4. This helps with bigger ships, since the smalls ones die to your lvl 2 towers anyways.


If you build 3 of the poison towers close to each other, they will even freeze Boss-Ships in place!

This is an example on how my towers look like in mid game:

The achievements come just by themself. Only a few of them need your help:
Cabin boy
Lose three times in a row
- This can be easily done by choosing any level and not building anything -> Then repeating the level twice more

On the edge
Complete a level with one heart remaining
- Choose level 1 on Hard -> Leave the first wave through completely
- During the second wave, count the first 7 enemies, let them through and build towers behind them to kill the rest

Slipped!
Allow only one enemy to attack the base's gates throughout a level
- SImple, let the first enemy go through and kill the rest on any level on easy mode

Man
Complete a level without using spells
- This is also very easy -> After the level where you got the fire spell, don't use him for one full level

Sniper
Kill 5 enemies in one shot
-This one is a bit tricky, since we only build ballistas. You need to play a level where you build lots of either sniper towers or Artillery. Your choice

As you can see in this screenshot, i've allready beaten Easy / Normal and am working my way through Hard now. This build works great for any levels!


If you have any questions or tips i might have missed, feel free to message me.

Special thanks to Kabn and adam1224 for proofreading the guide!