Tacitus on the Germanic peoples' sites of worship:
"They conceive it unworthy the grandeur of celestial beings to confine their deities within walls, or to represent them under a human similitude: woods and groves are their temples; and they affix names of divinity to that secret power, which they behold with the eye of adoration alone."
And Adam of Bremen's take on the same subject:
"[The Swedes have] a very famous temple called Uppsala, situated not far from the city of Sigtuna and Björkö. In this temple, entirely decked out in gold, the people worship the statues of three gods in such wise that the mightiest of them, Thor, occupies a throne in the middle of the chamber; Wotan and Frikko have places on either side."
These two accounts have come to represent the two sides in the debate of where Germanic rites of worship were held. The traditional belief, represented by Tacitus, is that the Germanic tribes of the Iron Age had a religion centered on nature, and on the fertile land. Thus, their religion was also practiced under the open sky, with the trees so vital to their view of the Cosmos forming a backdrop for their rituals of sacrifice to the spirits and to the gods.
In Sweden however, the existence of a glorious golden temple in Swedish lands has been too tempting a proposal to reject out of hand. Thus, the "Hall of Worship" view has ever had many Swedish adherents.
Interestingly, during recent years, archaeologists have uncovered a number of great halls in Scandinavia, some of them with clear religious significance, which seem to support Adam of Bremen's account.
So does this mean we have to consider Tacitus' account untrue? Of course not. To begin with, a thousand years separate the two scholars - a time frame in which much can happen to a religion as prone to change as the Norse one. Secondly, the existence of halls of worship doesn't in any way preclude outdoor worship, the two customs can coexist. Also, Adam writes of just one of a great number of Germanic tribes and groups, and as customs differ between groups today, so must they also have done during ages past.