Why did European walls evolve triangular star-shaped bastions?

Answer: To eliminate dead zones — each face covers the others

To make the fort visible from a longer distance — Visibility was never the aim; if anything, star forts sat low in the ground with their walls screened behind the glacis, precisely so cannon could not get a clear shot at them. The star outline exists so that every face is covered by fire from a neighbouring bastion.

To eliminate dead zones — each face covers the others ✓ — Correct! After about 1500, cannon could batter down the tall, straight curtain walls of the Middle Ages. Engineers answered with geometry: low, thick walls with triangular bastions jutting out at the corners, laid out so that every stretch of wall and every bastion face could be swept by fire from the neighbouring bastion. Wherever attackers approached, they were shot at from the side as well as the front. This 'trace italienne' spread across Europe for three centuries and produced the star forts of Naarden and Palmanova.

To deflect cannonballs with sloped angles — Slopes did matter in cannon-age forts, but not as angled walls that bounced shot away. The main walls were low and backed by earth to absorb hits, and outside the ditch lay the glacis, a gentle earthen slope that hid the wall from direct fire. The bastion's triangular shape is about lines of fire, not deflection.