A guess....I don´t know much about the sports jargon.

A little bit of context would help enormously!!!

My teenage son says there's the "home field" and the "away field". It's colloquial, of course, and without more context, we can only suggest things that may or may not fit.

Opposing territory, opposing teams, opposing sides, opposing players.

See photo:
http://www.vqorthocare.com/mitek/quickanchorplus.htm

... Inserter. Einführstab/***Varilla de inserción***/Outil d'insertion/ ... acoplada a una varilla de inserción. Nunca remueva una prótesis de voz e inserte ...
http://www.inhealth.com/pdf/37478-A_16Fr_Indw_Clin_PID.pdf

... in disguise: unyielding up front, swift to punish mistakes in opposing territory, pragmatic to a fault ... defensive system exposed by a cross-kick try to the outrageously athletic ...
http://www.buzzle.com/editorials/3-28-2004-52266.asp

I play sports

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Note added at 1 hr 3 mins (2005-02-04 04:19:07 GMT)
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enter opponent´s ground

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Note added at 1 hr 4 mins (2005-02-04 04:19:30 GMT)
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Confidence high

box = penalty box (area). Looks like a football (soccer) expression to me. You don't need to say "opponent's box" as it's understood as an attacking term.

Some refs from the UK:

Guardian Unlimited Football | Columnists | Moyes must give new ...
... Cahill gets in the box early and that moves another centre-half out of the way. ... Email
your comments to [email protected]. UP. ...
football.guardian.co.uk/Columnists/ Column/0,4284,1386622,00.html

Manchester United: Ron Atkinson: Why Scholes needs to be United's ...
football. ... The downside of playing Scholes in that position is that he gets in the
box far less than when he's operating off the front and for me that's a waste ...
www.manchesteronline.co.uk/.../s/64/64957_ ron_atkinson_why_scholes_needs_to_be_uniteds_middle_man.html

Newcastle United Football Club - Match reports - Leicester (a)
... "He gets in the box - he nearly got us a late winner last week. ... We're not
expected to win games of football at this club anyway'. ...
www.nufc.com/html/2003-12-26leicester-a.html

Check the Laws of the Game at www.fifa.com

There you'll find some FAQ which will show you a little bit the vocabulary used.