This experience report can also be found on the ISO website.

The idea of doing a semester abroad came to me early on, but it didn't make sense for my degree programme before my main studies. After initial contact with the International Student Office ISO (just go there and get friendly advice), I first contacted my ERASMUS coordinator, who promised me all the support I needed (in my case, Prof Martens). For chemists in particular, the only option is to complete the internships abroad that are otherwise required in Oldenburg itself and not to take part in any subject-specific lectures (at least this was not the case for me, as my Spanish was also more than poor). After 2-3 meetings with Prof Martens, everything was perfect, all the papers were signed and we were ready to go.
It is always highly recommended to make contact with either local students who were at the university in question (or in the host country) or international students from the university or host country where you would like to go.

When I arrived in Valencia, I was lucky enough to be picked up by my Spanish landlord (I had already found a flat in advance through a friend who was also in Castellón). Transport is otherwise not a problem, there is both the underground (€1.70) and a bus (€2.50) to the train station, from where you can get to Castellón within an hour on the so-called "cercanias" (€3.80). The classic way to start is to stay in a hostal ("Esperanza") to find accommodation, but this is usually quick and easy as almost all rooms are furnished.
Shared flats with Spaniards are recommended, simply to speak a lot of Spanish in everyday life.

In my case, I knew directly which professor I had to go to. As an exchange student, you usually end up in the "OCIE" first, where it's easy to get started and everything relevant is taken care of, from help with registering for courses and applying for a student ID card to help with any problems - and all in English.
Apart from that, it's a fairly new campus university on the outskirts of the city, but it's easy to get to on foot or by bus. The canteens are disastrous if you are used to Oldenburg standards, there is not much variation, and a menu for 4€ is not very recommendable in terms of price. However, microwaves are always available to heat up food you have brought with you.
The library is very modern with many, many workstations, there is also a large computer room and a Language Centre, language courses are usually offered before the semester as an intensive course with 5x3 hours/week and a semi-intensive course with 3x3 hours/week (Mon, Wed, Fri). The courses cost €55, which is ridiculously cheap for the number of hours, and also include a work component that you have to do outside of class (watching films, possibly reading books, "homework").
Working in the lab was good, I was ultimately flexible and my own boss, just as you would imagine. In some things, the lab lacked a "German structure", i.e. it was quite chaotic at times, but I could easily overlook that. I was very well received by everyone, at first I spoke English, but as time went on I spoke more and more Spanish, as my Spanish allowed me to.

Castellón is about 10 kilometres from the beach ("El Grao"), but you can easily reach it by bus, which runs every half hour. Castellón is located in the autonomous Comunidad Valencia, which also means that the 1st official language is "Valenciano" and not "Castellano" (i.e. Spanish). The languages don't have much in common (valenciano is the language that the Catalans have "stolen" for their Catalonia and call "catalán"), but especially at the beginning I found it difficult to hear not just one, but ultimately two foreign languages.
Castellón has about as many inhabitants as Oldenburg, but roughly only a third to a quarter of OL's surface area, meaning there are actually only multi-storey houses in the whole city, which are quite ugly and cause an acute shortage of parking spaces, but also give it a different atmosphere. The weekend runs from Thursday to Sunday, and there's a lot going on in the streets, even at night.

Spain has quite a different culture to us in Germany: lunch is hardly ever eaten before 2pm, between 6pm and 8pm there are the famous "tapas", small snacks, and dinner is only served from 9pm, with two hot meals a day as a rule. Olive oil is as much a part of the meal (whether for frying, as a dressing or otherwise) as the Amen in church, and there are also oranges and lemons in abundance, especially from the "orange country" around Castellón.
The city festival "Magdalena" in March is highly recommended, 10 days during which the university is completely closed and there is a lot going on in the city. Daily parades, processions, concerts, etc. make these 10 days a must for every ERASMUS student.

Personally, I spent more per month on average in Castellón than in Oldenburg (let's say around €700), but this is also due to the fact that an ERAMUS year is not a "normal" year, as it can happen that you go out 5-6 evenings a week. Food is a little cheaper overall, especially fruit and vegetables. Mobile phones are more or less the same as in Germany, there are a few big providers, but anything other than a prepaid card is hardly worthwhile (you might want to check whether you can "suspend" your contract in Germany for a while, O2 for example offers this).

The local AEGEE (www.aegee.org) regularly organises activities and weekend trips (Barcelona, Madrid, Granada, Salamanca, etc.) which are highly recommended, simply because you are guided by people who know the area.

A semester abroad is more than recommendable, you get to know a new culture and language or deepen your knowledge, you get to know many, many new people (from all over the world), and all this "alongside" your normal studies, which are usually continued. The EU also provides financial support!