The Space Link costs 440$ and is used to create space planes. These units fly about 2x higher than normal air units and can get over large hills than their air unit counter parts. Currently so far in the Conquerors games there are only 2 space units, The Mothership and the Space Fighter. The Space Link is a quite small and thin building so it is recommended to put it between defensive structures. Space units aren't completely worth what you pay as they don't dish out too much damage and they are squishy and can't take too much damage. However, swarms are perfect for destroying power plants, and only a stealth bomber is faster.