Spring (March to May) and mid-autumn (October to November) offer warm, pleasant weather and the famous orange blossom, ideal for the city without extreme heat. Seville is one of the hottest cities in Europe, with July and August often above 40C, while winter is mild, quiet and cheaper.
Petty theft is the main concern, concentrated at the cathedral queues, Santa Cruz, Plaza de Espana, the Setas and on public transport. Keep valuables secured and out of back pockets, and stay alert in dense crowds and at viewpoints.
Summer heat is genuinely extreme, regularly topping 40C. Carry water, use shade and sunscreen, avoid the midday sun, and plan the most demanding sightseeing for early morning or the evening.
Seville is generally safe at night, but the old town's maze of quiet, poorly lit lanes can be disorienting after dark. Stick to busier streets, keep your phone secure, and use a map to avoid wandering into empty alleys alone.