You can ride a cable car to 3,550 metres in ten minutes. The last 160-odd vertical metres are the difficult part, and it is not the gradient that is the problem.
Teide is the highest mountain in Spain. The official height is normally given as 3,715 metres, although some sources state 3,718. It sits in the middle of Tenerife, in a national park that receives several million visitors a year.
The permit
From the upper cable car station at La Rambleta a short trail called Telesforo Bravo leads up, named after the Canarian geologist. It is steep but only a few hundred metres long.
According to the national park's own regulations, Telesforo Bravo is the only path visitors are allowed to take up to Pico del Teide, and it requires authorisation obtained in advance. You are also not permitted to leave the trail at any point.
The number of places is small. No more than 50 people may be on the trail at the same time, and no more than 10 at a time on the final section that enters and runs along the crater rim. Walking all the way around the crater, or descending into it, is not permitted at all, because the area is classified as a reserve.
There are several types of permit. The standard one covers time slots between 9 a.m. and 5 p.m. There is also a mountaineering permit for between 6 and 9 in the morning, outside cable car hours, for those who want to see sunrise from the summit. In that case you have to be off the trail before nine.
Places are released a couple of months in advance and they run out. If you are planning a trip, this is the booking to make first, not the flight.
The caldera is bigger than the mountain
What really dominates the terrain is not the cone but the bowl around it.
Las Cañadas is a volcanic caldera measuring roughly 16 kilometres along its longest axis. Its southern wall still stands as a high rampart, while the northern side is missing entirely: it collapsed and slid into the sea around 170,000 years ago. Teide itself has since built up inside this damaged bowl.
It is unusually clear in elevation data. You can see the caldera wall as a sharp arc, the cone in the middle, and the lava flows that ran out across the caldera floor. On our Teide piece the cone sits in the centre and the wall in the south, which is the window where both relationships are visible at once.
Altitude that is felt more than it looks
One thing surprises many people: the altitude is noticeable. The cable car takes you from around 2,350 metres to 3,550 in a few minutes, with no time to acclimatise. Plenty of people feel dizzy at the top station, and the final vertical metres therefore feel considerably heavier than the equivalent climb would at home.
Take it slowly, bring water, and expect cold and wind even in midsummer. This is a high-mountain environment in the middle of a holiday island.
Afterwards
If you have been up there, it is the terrain you remember: the brown, almost lunar caldera floor and the cone rising out of it. That is also what the relief carries.
More volcanoes in the range? Read about Etna, where the highest point keeps moving between craters.