http://en.wikipedia.org/wiki/NetStorm:_Islands_At_War
Netstorm: Islands At War was a real-time strategy game released in 1997, and it was quite unlike anything that had come before or since. Set in a world of floating sky-islands, your goal was to storm the enemy island, capture their high priest and bring him back to your sacrificial altar to kill him permanently. You did this by building bridges in puzzle-game fashion, assembling paths from point to point using a pool of randomly assigned pieces, and could only attach end-points to other end-points or landmasses. You also defended and reinforced your extending bridge network by building defensive and offensive towers which behaved not unlike those we see in modern tower-defense games.
Alpha Available
http://risingstormhq.com/news.php