Xray - Because when I edit the scene and re-post it, it makes another scene to exist with the same title to be published on Algobox. After I delete that scene, Algodoo pairs the scene in my scene folder to the one I deleted.
Press space then press play. There are red buttons on the right. Click the button, and then a square will appear. Then, you put your cursor where you think your desired tower will perform best. After that, you click. Press [RETURN] after each wave, or to start the game. Click on a tower to see the upgrades and range, and press LEAVE if you want to leave the tower upgrades section.
You might want to play the original legacy (the Algodoo series) from CTD1 to CTD3 and CTD1-R. Search for "Circle Tower Defense" on Algobox, it is way better at telling you what to do than my scene.
ezpz - The circles have a varible telling how far are they in the course. The tower has a really fast spinning lazer in the middle that detects for the circle that has the most distance into the course. Supports circles speeding also.