This app was mentioned in 15 comments, with an average of 5.27 upvotes
Can't leave Robo Defense out of there. It was the first tower defense game I played back when I had a G1. Also the first app/game that I bought the paid version of.
I lost far too much time to Robo Defense.
https://play.google.com/store/apps/details?id=com.magicwach.rdefense_free&hl=en
Man it looks so dated already!
Personal opinion here, and I don't think I represent the majority. I've been playing TDs since the Warcraft 3 times (Green TD, Line Tower Wars, Circle TD, Gem TD, Ryoko TD, etc...), also played Sanctum 2 and Dungeon Defender for a while. Naturally, I tried some android TDs from the playstore only to get bored of them like 15 minutes later. The only android TD I got hooked up for hours was this one: https://play.google.com/store/apps/details?id=com.magicwach.rdefense_free
What I like most about TDs:
Building anywhere you want, because that way you can be creative with building mazes like spirals or long lines, in order to delay the mobs and have more time to hit them.
Multiplayer Support, because it's fun to strategize with friends. Or join a random lobby, and learn from people that already mastered the game (like I did back in the day).
Variance in the towers. There should be close range ones, long range, splash, slow, poison, flying etc... Accommodating the variance in the towers, there should be variance in the mobs with fast, armored, air, BOSS etc...
I like the TDs to be hard and fast paced. Like lose-in-the-3rd-round hard if you are a newbie. Because the harder it is, the more you need to think how and what towers to deploy. And of course, it's a lot more rewarding in the end.
Add a lot of upgrades. Green TD in Warcraft 3 had like 20+ upgrades to the first simple tower. Of course, you were just mindlessly hitting the upgrade button in the end. I would suggest like 5 upgrades to each tower. That way, the player also has the choice of spending money to upgrade certain towers however he wants. For example, you got a spiral maze full of cheap level 1 towers, and in the center of it you got this big splash level 5 tower that does most of the damage.
TL;DR I like TDs that promote creativity and lots of customization, instead of "build 2 archer towers in this 2 spots and you win this level" type of TDs, sharing the experience with friends/random people online.
Robo Defense has a free demo with the open map you described, first game I bought on my HTC magic.
Robo Defense has been my goto for a while https://play.google.com/store/apps/details?id=com.magicwach.rdefense_free
The above and Dungeon Warfare 1 is also fun (Paid). I use to play this a while back and really liked it Robo Defense
I used to play this one a couole years ago, not sure about blimps but the rest sounds about right
https://play.google.com/store/apps/details?id=com.magicwach.rdefense_free
Robo Defense FREE | 4.2 rating | Free | 10,000,000+ downloads | Search manually
> Robo Defense is the ultimate portable tower defense experience. Featuring open maps, upgrades, achievements and nice graphics. The free version features 1 map with 11 difficulty levels. A full version is also available with more maps ...
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Robo Defense Free is bit older but I like free form of placing towers.
Usually it's Facebook and Twitter, but when I want a fun way to kill the battery, it's Robot Defense.