If you feel like you just aren't earning enough city cash, here is a way earn it quickly and easily.

The short version is this: Replaying the Consecrated Ground map still earns 5000 city cash. Given that it's a short mission and earns a lot, it's a good way to quickly earn some money.

To be able to make full use of this strategy, you'll need to have completed the Consecrated Ground tile before, and likely should have both Super Monkeys and Apprentices upgraded to tier 3s.

I have three Super Monkeys and only one Apprentice - it's likely possible with only two Supers, but I have not tested it.

The Consecrated Ground mission is a ten round mission where only Apprentices and Super Monkey is allowed.

The map itself is a short cross between two paths, like Main Street for those of you who've played BTD5.

The strongest bloon that can spawn is a BFB, and most of the bloons will be ceramics or MOABs. Camos can spawn, but personally I've only ever seen weak camos and only on round one.

To find the mission, you need to enter the "My Tracks" menu.
The "My Tracks" is found by clicking on the monkey button in the lower left, the same place you'll find the friends list, MvM, and Contested Territory.

Once you've entered the track selection, scroll to the bottom with the special tiles and select it, then scroll to the right to see the map.
Note: If you can't select the track, you probably haven't found the tile on the map yet, and need to capture it first. If the tile is incredibly far away from your city but you still want to earn some money, you could consider doing some of the less efficient missions (some still earn 1250 or 1000 cash) or wait until you can fight your way to the tile.

With only two towers, strategy is limited.
All towers should be placed as close to the map as possible, to maximize popping power.

The Super Monkey should be the bulk of your defense. I prefer to use a 3/2 in the lower right of center to start, or a 3/0 if camos spawn round one.

If the camos are pinks or below, you can sometimes get away with just a 2/2 apprentice. If the camos are stronger, I tend to use 2/3 apprentice, but in some cases you can barely survive with a 3/2.

Note that to get a 2/3 or a 3/2, you either have to buy mid-round or use a weaker super monkey to start. One possibility is getting a 2/x super, using an apprentice to pop the camos, then selling the apprentice for a 3/x super, or keeping the apprentice but using a 2/3 super.

As the mission goes on, you'll want to buy more super monkeys - two or three should be as much as you can afford in one attempt. I like to get 2/3 supers targeting strong, but I really don't think it matters much what you do past the first couple rounds. The mission arguably gets easier past the first rounds - you earn a lot of money in this mission, and your defense should get much stronger quickly.

By the end, your defense will probably look something like this:

And that's it! Every time you complete the mission, you earn 5000 cash. With a few rounds of this and some luck, it should be easy to earn lots of money for your growing city. Good luck!