If I remember correctly I think this map can be done with only lasers and blasters. Maybe some missile towers are needed in the end when money is not an issue.

I call the bottom part of the "Y" a tie, where joint section is the tie-knot.

1) Try starting off with two lasers beneath the "tie-knot". These can later be upgraded 1 or 2 times.

2) Then continue building blaster towers around the "tie" to force creeps down in the middle.

3) For the second or third wave you might need to add a temporary blaster tower in order to shoot creeps that will slip through. From the beginning you can't afford it so this has to be done when money drops in. That tower can/must later be sold in order to afford a tower on the other side.

4) Remember to build blasters forcing creeps into the "tie", when building blasters along the "tie" creeps will eventually take another route above.

5) At the bottom of the "tie" you can place one or two lasers that you upgrade a lot. You can also extend the tie further down before forcing the creeps up, this requires you to place the two lasers further down and not immediately after the "tie"

6) Then build blaster towers upwards around the tie again forcing creeps to pass blasters a second time.

This is how I do it. I haven't found any good way to get a lot of multipliers on this level. Maybe someone else has a suggestion?

Regards SuperB