Edit: I think the question is best answered by the semi-official English translation, offered by the Bundesministerium der Justiz (German Federal Ministry of Justice):
(1) Whoever unlawfully enters the private premises, business premises or other enclosed property of another, or closed premises designated for public service or transportation, or whoever stays there without being authorised to do so and does not leave when requested to do so by the authorised person incurs a penalty of imprisonment for a term not exceeding one year or a fine.
Given the debate regarding one definition, I would like to take the history of the law in question into account and give more linguistic and legislative sources.
The term befriedet has been in § 123 Abs. 1 StGB since the very first version of the StGB from 1872. In the DWB entry that was published in 1853 – only 19 years prior –, we read:
BEFRIEDEN, protegere […], wurde zumal auf das hegen und schirmen des landes und feldes gegen feinde und schädiger angewandt […]
BEFRIEDEN, protegere […], was especially applied to fostering and sheltering the ground and field against enemies and wrongdoers […]
Accordingly, befriedet in the sense of § 123 Abs. 1 StGB must be understood as an estate that is somehow delimited and protected from trespassing, that can be by a wall, by a fence etc. – maybe enclosed would be an appropriate English term.
At the time of legislation, befrieden was probably more an everyday language term than a terminus technicus/legal term. The more recent legal definitions, e.g. by the German Federal Court, that were cited by other answers and comments are a specification of that 'natural' meaning, but they didn't re-invent it. First of all, befrieden meant to protect a piece of land. In the context of the formation of the trespassing law in the German penal code of 1872, that must necessesarily mean by an enclosure.
§ 134 Abs. 1 of the penal code of the GDR used the term umschlossenes Grundstück (literally 'enclosed estate') instead which means the same.