Treat the Walls as an outdoor walk, not a museum visit.
There are long exposed stretches and many stone steps. The official operator specifically recommends checking the weather forecast, wearing appropriate clothing and footwear, carrying drinking water, using a hat or cap and applying sunscreen.
Morning is the easiest default.
If your priority is comfort, an earlier visit usually gives you the largest margin before the strongest heat. It also leaves the rest of the day free for the Old Town, sea or lunch without rushing the circuit.
Later can be beautiful, but check the clock.
Softer light and lower temperatures can make a later walk appealing. Opening hours are seasonal, however, and the official operator defines the published closing time as the last admission time. Give yourself enough time for the circuit rather than arriving simply because entry is still technically possible.
Crowds change how long the walk takes.
Narrow sections, tower stairs and popular photo points can slow the circuit. In peak periods, build more margin than you would on a quiet spring or winter day.
Quick answers
Is Dubrovnik Walls too hot in summer?
It can be demanding in strong heat because much of the circuit is exposed. Earlier or later visits are usually more comfortable.
Can I buy water on the Walls?
Availability can change, so the safest plan is to carry water with you from the start.
What should I wear?
The official operator recommends appropriate clothing and footwear, plus water, a hat or cap and sunscreen.
References: Official City Walls visitor rules & FAQ