Easy "Builder (Level 14)" achi...
	Easy "Defender IV" achievement...
	Easy "Master Strategist (Level...

Use the "Barricade" card. The obstacles will count as towers. This allows you build twenty towers for 200 Prime. If you are short, sell one of your main damage towers to build more barricades. Do this while the game is paused at the last enemy.

Building towers is limited by your starting funds and the number of available tower squares. You can only save your progress towards the total by completing a level. Select a quick level that has enough Prime to fill a lot of squares, and build the cheap "Barricade" tower. Consider doing this on level 13 "At the Andornian Border" where you can fill all 58 tower squares in about two minutes and allows you to construct your base at the spawn point. Another choice is level 14b "Flower Queen (Boss)", where you can construct up to 48 towers in about 45 seconds then defeating the Boss with a level 1 sun tower to complete it.

The description is misleading; you actually cannot build any Poison Towers to qualify.