A first half penalty helped Newtown get through a tricky second round tie against a plucky Penybont outfit.
Ethan Jones missed a penalty before tapping home the re-bound to give the Robins a half time lead.
In the second period Newtown failed to put the game to bed, and the visitors went close to levelling late on but Newtown hung on.
Chris Hughes opted to mix his squad up for their second game in four days, and ahead of a long trip down to Llanelli on Friday night.
Dave Jones, Ben Bailey, Neil Mitchell, Steve Leslie and Nick Rushton were all rested, as Jojo Harries, Jack Perry, and Tom Goodwin were handed their first starts.
Jones and Sam Phillips were also recalled to the first 11.
Newtown began brighter against their inform Welsh League opposition, with Alex Fletcher volleying wide from a tight angle in the opening moments.
It was clear they would have the majority of the early possession, as Penybont were willing to sit in and soak up the pressure.
And they did that with success early on, as the Robins failed to any joy when pressing for that final killer pass.
Despite the lack of possession, Penybont had the first quality opening but they failed to capitalise on a Robins error.
Phillips conceded possession and a through ball saw Marcus Griffiths go one on one, but he failed to get a shot off.
Almost immediately Jones went close at the other end as Fletcher found him in the box, but Rhys Wilson palmed is effort over.
From the resulting corner the Robins did take the lead as Jay Denny’s header came off an arm, and the referee pointed to the spot.
Jones’ firm penalty was saved by Wilson, but came back to the forward who tapped the ball home.
Newtown went after a second with Phillips looking very lively in the middle of the mark, however they were just struggling to find that defence splitting ball.
A superb ball five minutes before the break from Phillips almost found Luke Boundford, but Kane Owen scrambled back to make a vital interception.
Penybont went close to an early equaliser in the second period, but a dangerous cross was cleared at the last second.
Then Newtown should have been two ahead as Jones had a shot blocked, before a Steve Leslie through ball sent the striker clear, and he cut inside only to be denied by Wilson.
With the game still at 1-0 Penybont were still in the game, and they had a penalty shout turned away.
They were beginning to ramp up the pressure just after the hour mark and getting into good areas but failing to find an end product.
The Robins were inches from wrapping the game up 20 minutes from time as a cross fell to Leslie, but his goalbound effort was hooked off the line.
Then seconds later Nick Rushton’s cross found Phillips, however he could only loop a header over.
Penybont were still coming forward as Curtis Hutson lashed high and wide, before a fierce Luke Borrelli drive was held by Jack Perry.
The went even closer with five minutes to go as Perry had to be alert to tip over a smart flick from Borrelli.
Late on Owen saw a free kick deflected as the Robins held on to secure their route to round three.