another improvement by pyrogear

here's another quote form pyrogear on how to improve the 171 strategy.

"A slightly better build is to full upgrade the initial cannon for level 3. After levels 3 and 4, you have less gold, but by the end of 5, you will have 2 extra gold, and the same full cannon. There is some variation in the game. The creeps don't move uniformly, and the cannons get off shots in different places, splashing different places. With one cannon on level 6, I have occassionally had no leaks, and had as many as 5 leaks with the same placement. One cannon will also sometimes be sufficient for level 7, but in both cases, usually not. Another improvement is to just have the 1++, 1+ for level 10, then add 2 fully upgraded arrow towers for the boss (as Jsmith said). Level 11 only needs 2++, saving a little more. The second boss can be killed with one leak with 7 cannon and 5 arrow towers. The last boss with one leak by adding 7 arrow towers. "