I don't like the idea of a base needing a watch stander 24/7. It creates a situation where someone, or a group of players, can't do anything in the game except to keep watch for attackers. The way Eve Online got away with this was reinforcement timers. The attacking force would attack the shields of a large structure, and then it would become invulnerable for a set time. This allowed players who may be in another timezone to mount a reasonable defense. It worked for that game and it may be worth thinking about!