Drawbacks of relying on static defences
Ogame is no longer a new game. That means on most worlds there is a core of experienced players who have been around long enough to build up huge fleets - fleets more than capable of knocking out any static defence you might hope to build. And because of the ways conflicts are calculated, they will not even suffer appreciable losses. Multiply this factor by the way all of these players are banded together in alliances.
Forget all the talk of "Turtleing." The only defence that these experienced players fear is nukes. Nukes cut through their fleets and destroy their constructions.
Beware, however. Use of nukes will cause established alliances to freak out and put enormous combined efforts into eradicating you. They will use any means at their disposal - fair and unfair. Be especially careful in correspondance with them as they will try and trick you into saying something that contravenes game rules - and they will have you booted.