In Beijing, China, eight luxury residential towers that were designed by Steven Holl Architects are connected on the twelfth through eighteenth floors by enclosed glass and steel bridges.

This sentence is a complex sentence.

Explanation:

A complex sentence is a sentence that contains an independent clause and one or more dependent clauses. An independent clause can stand alone as a sentence, but a dependent clause even though it has a subject and a verb cannot stand alone.

The sentence contain one independent clause and one dependent clause,

the independent clause is in bold below;

eight luxury residential towers that were designed by Steven Holl Architects are connected on the twelfth through eighteenth floors by enclosed glass and steel bridges.

it has a subject and a verb.

While the dependent clause is in bold below;

In Beijing, China,

it does not make any sense on it own.