Learn Portuguese (Portugal): Antigamente, não havia tensão geral nesta assembleia. - In the past, there was no general tension in this assembly.

The word antigamente (formerly / in the past) sets the scene for a continuous, ongoing state rather than a single completed event. When describing how things used to be or what generally existed over an indefinite period of time, we use the imperfect tense havia instead of the simple past houve.

Could I say não tinha tensão instead of não havia tensão?

In spoken, informal Portuguese, people sometimes use the verb ter (to have) to mean there is or there was, so you might hear não tinha. However, não havia is the grammatically correct way to express there was not, and it is heavily preferred in Portugal for both writing and careful speech.

Why is the word nesta used here instead of na?

Nesta is the contraction of em (in) and the feminine demonstrative pronoun esta (this). We use it here because the sentence specifically refers to this assembly. If you used na (em + a), it would just mean in the assembly.

Why doesn't the word assembleia have an accent mark?