The original Bloons challenged players to chuck darts from a fixed position at a level full of balloons and obstacles in your path. Leveraging predictable physics, immediate comprehension of the game mechanics and the fun that is popping balloons, it was deservedly successful. Time marches on and the concept has pretty much run its course - which is why the developers have branched out, re-imagining the concept as a tower defense game.
With snaking levels that look something like Zuma, you'd be forgiven for your immediate impressions being that Bloons TD is a clone of that puzzle franchise. Aside from the first impression, however, Bloons TD has very little in common with that concept.
Bloons TD doesn't mess with the basic premise of tower defense much, giving the player immediate access to a bunch of different monkeys (yep, monkeys) who are ready and willing to throw darts at the horde of approaching bloons. Why they don't just buy a fan is a question for another day.
The bloons vary in colour, each of which indicates its behavior: yellow bloons are fast and burst into blue bloons, blue bloons burst into red bloons and red bloons simply pop - ending the sequence. There are also bloons that are immune to sharp things (comically named Lead bloons) bloons that are immune to explosions, and more.
To tackle the increasing complexity of the incoming rubbery horde, you have a dizzying array of towers at your disposal. Starting with the basic dart thrower, there are a host of options available - should you have the cash to spend. Towers each have their own strengths and, true to the genre, deploying a variety wisely is the key to success. Knowing what to put where (hint: go for the choke points) is everything.
Towers aren't just deploy-and-forget - each has four upgrades available, split into two different trees. Typically what you'll be doing is increasing things like range, attack speed, number of targets affected - etc. But there are some surprising features available too - and you can buy them all, if you have the cash.
But it's not all about towers, with Bloons TD adding one-round deployable objects into the mix. These special objects can only be deployed on the road (unlike towers, which can only be deployed off-road) and at the end of the run, any that weren't consumed are destroyed. These unique objects are really intended to catch bloons that escape your net of towers, making their optimal usage time while the round plays out - typically you won't even know you need them until the bloons escape the range of your towers. Combined with the fact that the bloons move very fast and that controlling a cursor with any combination of PSP controller input is very difficult, this inclusion can only be considered "interesting" - a rating which ranks a couple of rungs below "successful". It seems like a cool mechanic but it's somewhat wasted when combined with a virtual cursor control scheme.
Popping bloons is what brings home the bacon (called, simply, "money") - and you're going to need cash to feed your insatiable appetite for towers and upgrades. It's in constraining the resource that Bloons TD dials up the difficulty - selecting higher difficulty levels increases the cost of items and upgrades, which in turn results in you having less ability to deploy objects. As levels advance in rounds, the net effect of this "resource tweak" is that you need to be unimaginably clever in how you spend your money. It also means that the game has a great difficulty balance, allowing newbie players to see most of the game while also providing some serious challenge for those that have the stones to tackle the higher difficulty levels.
The control scheme essentially puts the player in direct control of a virtual cursor, the bane of all console RTS games. Fortunately, the bulk of the time you need to use it, there is no pressure from an approaching wave of enemies as you call them (with Triangle) when you're ready. As already discussed, however, it does somewhat limit the utility and usability of the consumable / track-deployed weaponry. A grid-based system would have better enabled this type of functionality.
Additionally, precisely placing towers can be seriously challenging. The locations where you can deploy towers will vary, depending on the size of the tower you're trying to deploy. The cursor moves very quickly and, even when holding Square to slow it down, you'll often find yourself either going back and forward trying to find that one pixel you can use or giving up and deploying to a strategically weaker position. Again, a grid system would have completely solved this.
Graphically Bloons TD can only really be described as acceptable. With something so comical as monkeys popping balloons, some serious opportunity was missed in all aspects of the presentation. The interface is also super kludgy, with poor flow through the level select system and generally amateurish presentation throughout. It's never so bad as to cause any real issues but it's also super obvious - even to people that aren't picky game reviewers.
The sound is similarly disappointing, with some events not appearing to have any sound whatsoever. Only the balloon popping has any real satisfactory kick to it, which again is a crying shame (and, possibly, a crime) given the great material the developers had to work with here. It's monkeys popping balloons, people... it writes itself!
Gripes aside, Bloons TD is a very good tower defense game. It seems a bit awkward at first but if you persevere you'll find yourself hooked. There's loads of it too, with heaps of levels and a lot of play needed to unlock them all. It's not perfect but it's more than worth the price of entry - here's hoping there's a sequel because there's plenty of cool stuff that can still be done with the premise.