Croatia is an Eastern European country with a long coastline on the Adriatic Sea. Encompassing more than a thousand islands, it's also crossed by the Dinaric Alps. Its inland capital, Zagreb, is distinguished by its medieval Gornji Grad (Upper Town) and diverse museums. The major coastal city Dubrovnik has massive 16th-century walls encircling an Old Town with Gothic and Renaissance buildings. The Croats arrived in the area of present-day Croatia during the early part of the 7th century AD. They organised the state into two duchies by the 9th century. Tomislav became the first king by 925, elevating Croatia to the status of a kingdom. Most of Croatia has a moderately warm and rainy continental climate. Mean monthly temperature ranges between −3 °C (27 °F) (in January) and 18 °C (64 °F) (in July). Tourism is a significant source of revenue during the summer. The bulk of the tourist industry is concentrated along the Adriatic Sea coast. Opatija was the first holiday resort since the middle of the 19th century. By the 1890s, it became one of the most significant European health resorts. Later a number of resorts sprang up along the coast and islands, offering services ranging from mass tourism to catering and various niche markets, the most significant being nautical tourism, as there are numerous marinas with more than 16 thousand berths, cultural tourism relying on appeal of medieval coastal cities and numerous cultural events taking place during the summer. Inland areas offer mountain resorts, agrotourism and spas. Zagreb is also a significant tourist destination, rivalling major coastal cities and resorts.