Samarbeidsspillet er lett, bortsett fra at tidsbegrensningen er kort.

Because Norwegian usually writes compound nouns as one word.

Samarbeidsspillet is built from:

• samarbeid = cooperation
• a linking s
• spill = game
• -et = the definite ending

So the whole word means the cooperation game / the co-op game.

The double ss is normal here: one s is the linking sound, and the next s belongs to spill.

Those endings show the definite singular, but the ending depends on the noun’s gender.

• et spill → spillet
• en begrensning → begrensningen

So:

• spill is a neuter noun, which gives -et
• begrensning is a common-gender noun, which gives -en

This is one of the big differences from English: Norwegian often puts the at the end of the noun instead of using a separate word.

Because the sentence is talking about specific things:

• a particular game: samarbeidsspillet
• that game’s specific time limit: tidsbegrensningen

So the speaker is not talking about cooperation games in general, but about one known game and its time limit.

It means except that or apart from the fact that.

It introduces an exception to what was just said:

• Samarbeidsspillet er lett = the game is easy
• bortsett fra at tidsbegrensningen er kort = except that the time limit is short

So the overall idea is: the game is easy, with one important exception.

Use bortsett fra before a noun phrase, and bortsett fra at before a full clause.

For example:

• bortsett fra tidsbegrensningen = except for the time limit
• bortsett fra at tidsbegrensningen er kort = except that the time limit is short

In your sentence, the part after it is a full clause, so at is needed.

Because at introduces a subordinate clause.

In subordinate clauses, Norwegian does not use the normal main-clause verb-second pattern. The subject stays before the verb:

• at tidsbegrensningen er kort

A useful comparison:

• main clause: Tidsbegrensningen er kort
• subordinate clause: ... at tidsbegrensningen er kort

The difference becomes clearer with words like ikke:

• ... at tidsbegrensningen ikke er kort nok

It is another compound noun:

• tid = time
• begrensning = limitation/restriction

So tidsbegrensning literally means time limitation, and in natural English that is often simply time limit.

Then -en makes it definite:

• tidsbegrensning = a time limit
• tidsbegrensningen = the time limit

Because these adjectives happen to have the same form in common-gender and neuter singular.

So you get:

• et lett spill
• en kort tidsbegrensning

And predicatively:

• spillet er lett
• tidsbegrensningen er kort

They do change in plural and some other forms:

• lette spill
• korte tidsbegrensninger

So the lack of visible change here is normal.

Here it means easy.

Lett can mean different things depending on context, including light in other situations, but with a game it is naturally understood as easy:

• et lett spill = an easy game

So the context tells you which meaning is intended.

Yes, you could say:

Samarbeidsspillet er lett, men tidsbegrensningen er kort.

That is grammatical, but the nuance is slightly different.

• men = but; simple contrast
• bortsett fra at = except that; points to a specific exception

So bortsett fra at sounds more like: everything is fine about the game, except for this one issue.