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.
