Bloons Tower Defence 4

Bloons Tower Defence 4 is quite a good tower defence game. To be honest I'm not sure how it compares to BTD1, BTD2 or BTD3, but I will endeavour to play them to find out. I think the most surprising thing about this game is that it's based on possibly the most dull game in the world - Bloons. All things considered though, it's quite entertaining to build you way up through the different available towers, bursting your way through progressively more resilient levels of balloons.

GemCraft Chapter Zero

One of my favourite games from last year was the addictive tower defence game GemCraft Chapter One. This year the makers have created the prequel to it - Chapter Zero. If you're anything like me then you'll have forgotten that the original game was loosely fitted around a storyline - and this game is a continuation of the same storyline backwards. I for one was certainly wondering how all these gems obtained magical powers and this instalment sheds exactly no light on my puzzlement.
Anyway, Chapter Zero is very similar to Chapter One, but with a load of new levels, a load of new enhancements and a load of new bonus towers on each level.

GemCraft Tower Defence

Similar to Desktop Tower Defence this game is based around defending against waves of enemy minions. In this game every tower can be made into a weapon by placing a gem on top of it. The various colours of gems have different powers, and you can combine them to make gems with mixed colours. The gems also have different strengths depending on the shape - by saving up you can purchase more powerful gems.
There are lots of levels, and the aim is to complete them all above a certain level, which I estimate would take days. Luckily you can save your progress and come back to it later.

Desktop Tower Defense

Desktop Tower Defense is a clever game where you have to create and maintain a field of towers. Periodically you will be invaded by a swarm of little critters and if any of them get through your `tower defense' field alive then you lose a life. You can upgrade your towers by clicking on them and clicking upgrade.

There are full instructions at the bottom of the page.