Grave Defense is probably the best I have ever played, and it is ported to everything. You can find copies on ios, androis, playbook, and webos. Just letting you know incase you have other devices types.
Yeah we really are missing a few good tower defense games. I know plants vs zombies is coming but I want a different one to. Like Jelly Defense (amazing TD game but already asked them and they are too loyal to stupid iOS) and one of my favorite Field Runners
I've actually been playing a free (flash) one on Kongregate called Kingdom Rush. It's also available as an app for android and iOS. It runs decently and you can make it full screen (with black bars on the sides) by double tapping on the flash element.
Kongregate is a great site for free flash games, so definitely take a look around there. Just keep in mind you may not be able to play them all as some require keyboard input to move around.
Have both Lonely Turret and Grave Defense, love them both for different reasons. LT feels much more hands on and involving, I like how it's constant interaction kinda like PvZ. GD is a really good take on the more classic TD type game.
Grab em both! Should be less then $5 for both of them I think...
Before the OP I downloaded Jungle Defense for the Playbook. It is OK but nowhere near as good as others similar games I have played on other platforms, hence the question.
I am in the process of downloading Grave Defense and will certainly download the others suggested so far.