When Colwyn and his allies see the Black Fortress they intend to attack he says, "It's almost dawn - we must get inside before the twin suns rise!" But the sky is blue with white clouds. When they then ride up to the fortress, it repeatedly alternates between twilight and daylight.
When Ynyr comes back from the widow of the web and declares "At sunrise the Black fortress will appear in the Iron desert!" he falls over. As he does so he damages the 'rock' he is standing on, revealing the polystyrene it's made from underneath.
In the first shot of the Black Fortress, the film is being played backwards as shown by the smoke from the burning torches trailing in the wrong direction. Later the same shot is re-used but played forwards, showing the Slayers returning to the fortress after they have kidnapped Princess Lyssa.