Learn German: Der Winter war sehr lang und kalt. - The winter was very long and cold.

In German, the seasons are masculine nouns, so they use the article der. While English often drops the word 'the' when talking about seasons in a general sense (like 'Winter is cold'), German usually keeps the definite article.

Are all the other seasons masculine too?

Yes! All four seasons in German are masculine: der Frühling (spring), der Sommer (summer), der Herbst (autumn), and der Winter (winter).

Why don't the adjectives lang and kalt have any special endings here?

Adjectives in German only get extra endings (like -e or -er) when they are placed directly in front of a noun, such as in ein langer Winter (a long winter). Because lang and kalt come after the verb war, they stay in their basic dictionary form.

I usually see the spoken past tense used in everyday conversation. Why is war in the simple past?

war is the simple past (Präteritum) form of sein (to be). While spoken German strongly prefers the spoken past (Perfekt) for most verbs, sein and haben (to have) are major exceptions. It is much more natural and common to say war instead of the Perfekt form ist gewesen.