Bloons is a bit weird in the tower defense genre. You don't go through a specific set of maps, instead you can attempt any of them at any difficulties you've unlocked. You unlock better towers with a global level - and then you buy them during maps with cash you get from popping balloons. Some of the maps are weird and there are so many towers that it feels a bit like diluting the overall use of each types. I still had a fun time with it, but the lack of a clear progression made me stop after a while.
You start at level 1 with only one tower type unlocked and you get experience both for yourself and for your towers by popping balloons. The first tower type is quite simple, it shoots darts at them; Then you can upgrade it to shoot more darts or too shoot faster. You get boomerang towers that fire arcing projectiles. Snipers that can shoot from everywhere on the map, glue towers and ice towers to slow down the balloons - and pop some of them as well - and it culminates with a super monkey that fires eye beams really, really fast. It kinda should've stopped there, but no. Boats that can only be placed in water, planes that do weird 8 maneuvers around where you place them, mortars that aim a specific spot, miniguns that fire at the last place you've tapped - even to select a tower or buy an upgrade - and so forth. I don't mind the odd money-making tower or the tower-boosting tower, but there are way too many of them. Especially with eight different upgrades per tower type.
The balloons are fine as an enemy. There are normal balloons that pop if you hit them, others have multiple 'layers'. Some are made of ceramic or metal and need special attacks, some go faster, some contain a bunch of smaller balloons, some are invisible, some are really really tough and are full of other tough enemies. It's typical tower defense fare with the balloon twist. That being said, there is no real progression in enemy strength as you play through the levels. That's the thing I disliked the most about this game, you could start on any map since a bunch of them were unlocked when you start, and each map is more or less similar. You start with easy balloons and it progresses to tougher enemies. Harder difficulties have more waves, but you don't unlock anything by beating them. I wish you had one map to start with, then you unlocked another, and then another.
The maps themselves are a bit confusing since they don't make it clear what paths your enemies are going to take. Sometimes you even have to infer from the map title - like 'Three Turns Around' - to determine movement patterns. But otherwise, there is no penalty to restart and you keep leveling up and unlocking new towers and upgrades, so even if you don't get a layout of one level, you can always restart. There's also a system where you use 'special' money you get from completing levels to buy even more upgrades for your tower. I enjoy upgrade paths, but this is a bit much.
This was my first Bloons TD game, and I had fun with it! Aimless fun, perhaps, but fun regardless. If you enjoy Tower Defenses and/or monkeys popping balloons by throwing darts at them, this might be for your iOS device!