OK, there's some debate at work whether it's fun or not, but I liked it. It's called "Tower Defence" and used to be found at:

http://www.jeannettevejarano.com/tower-defence.swf

although their bandwidth limit seems to have been exceeded.

It also appears to be available at:

http://en.t45ol.com/play/2541/tower-defence.html

but with loads of ads and pop-ups.

Enjoy!