Utumno, the strongest fortress every built?
 

Since the Valar in their first engagement with Morgoth, they spent years laying siege on the Utumno, before finally capturing Morgoth. So why was Morgoth able to hold out on the fortress that long? Did he cast a strong protection spell over it, like Melian did to Doriath?

Utumno held out for so long because it was vigorously defended, likely both physically and preternaturally though we have little detail, and was designed to be defended. As to whether it was the strongest fortress in Arda, I might guess that some might nominate Valinor, protected by an entire range of the tallest mountains in existence. And, like any fortress, no impregnable.

Melkor had also, seemingly, not previously wasted so much of his native potency as he did in the Battle of the Powers; indeed, 'Myths Transformed' suggests that the very act of empowering his servants in the defence of Utumno (or beforehand, such as in the destruction of the Lamps) was perhaps the first great reduction of his strength, with both Manwë and Melkor himself 'amazed' at this weakening: "he has now less personal force than Manwë, and can no longer daunt him with his gaze."

I think it's all relative to the strength of the defender's opposition, however.