It's taken me a couple weeks to beat it, but I was finally successful.
Each regular monster will yield 3 gold. The "bosses" yield 30 gold.
This isn't remotely enough income to build the upgraded towers.
The single greatest source of income is the interest on your unspent gold.
At higher levels, that will yields more than 400 gold per screen.
You need a tight strategy and a little bit of luck to pull it off.
The original strategy used exclusively Arrows because of their price. With the adjustments to 1.3, we need to mix up the tower types a lot more.
If we build enough towers to guarantee all the monsters are stopped, we will run out of gold in the later levels. So there is some luck involved. (At least for me.)
We can minimize the luck factor by building pretty conservatively and letting that gold carry with its compounding interest.
For the first 10 levels, we will generally keep the budget under 40 gold.
Level 1: Tap To Defend
Just building 3 towers.
By placing each tower in a "corner," each has maximum coverage. With this placement, you will clear the wave over 90% of the time. If we miss a monster, start over. Compounding interest makes losing a life in the early levels a disaster.
Level 2: The First Swarm
Three more towers.
I think I've only lost a life once with this configuration. Again, if you lose a life, start over.
Level 3: Slow but Deadly
Here's where we had to significantly change the strategy from before. Since the monsters are slow and usually densely grouped, a Bomb is the most cost-effective way to deal with them. One Arrow tower at the end will have good coverage to demolish any stragglers.
Level 4: 2 Heads > 1
Instead of upgrading towers, we will build our usual three Arrows.
Fill in between Arrows we built on level 3, but let the bottom one get a little extra coverage.
Level 5: The Demon Within
3 more Arrows.
Fill in the row of arrows on the right.
Level 6: On Fire
I actually follow the advice here and build 2 Water towers.
This takes us over budget, but I will usually lose a couple lives if I only build one Water.
Level 7: Bat out of Hell
Here's the biggest break from the old strategy. We need the evenly distributed towers to take out waves of fast units, but each individual tower won't do enough damage to the Hellbat to take it out. The Hellbat is worth enough gold to make it worth our while, so we will upgrade the first Water tower.
Level 8: Go Green
Keep your Halo for interest upgrade later.
Because we had to overbuild last wave to take out the Hellbat, don't build anything this level.
Level 9: Better, Faster, Fuzzier
Back to the usual 3 Arrows.
Leave room next to the track for Ice towers later.
Level 10: Whip It Good
A second Bomb and an Arrow.
With any luck, you should have 330+ gold and all 20 lives at this point.
If you have less than 300 gold at this point, I'd recommend starting over.
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