The match at Richmond Park kicks off at 7pm.
The winners of tonights match will face TNS in the semi finals and guarantee themselves £25,000.
Paul Wilkinson will be in charge of the squad and he hopes to have some first team faces available to him but injuries to senior players makes that unlikely.
We could see games for new signings David Forde, Iwan Redan, Matt Green and James Simmonds and Welsh U21 players Joe Jacobson, Chris Gunter and Darcy Blake could feature.
There's no knowing though just what team will turn up. Carmarthen knocked City out at this stage last season and will be looking to scalp us again.
"It's a tough ask," said Carmarthen boss Mark Jones. "OK we might be playing a reserve team, but it's a reserve team of a club that could be going up to the Premiership in a few months."
Paul Wilkinson said
"We've got three or four injuries so the amount of first-team squad members involved will reduce.
"It's a big game for us and it'll be difficult. We played Carmarthen in pre-season and last season so we know how difficult a place it is to go to.
"We don't go there without showing them a lot of respect."
"It's a big opportunity for the some of the younger boys who will come in," Wilkinson added.
"We'll be looking for them to step up and take their chance.
"The likes of Darcy and Chris have taken their opportunities when they've been involved with the first team.
"That's been good for them and for the younger ones to see their mates have progressed through."