First half strikes from Julie Blakstad and Laura Coombs, as well as an own goal, set the tone for a dominant display at the Academy Stadium.
City continued to press forward in the second 45 with Castellanos and Blakstad both going closest to adding to our advantage.
But the only addition to the scoreline after the break came from the visitors when Lee Geum-Min scrambled a stoppage time consolation over the line.
Nonetheless, Gareth Taylor's side moved back to within touching distance of the WSL top three as our fine run of form continued ahead of next Sunday's crucial showdown with Manchester United at the Etihad Stadium.
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City had averaged just shy of five goals per match in our seven previous home WSL meetings with Brighton and didn’t have to wait long for the breakthrough in our latest encounter.
Kelly had already gone close to the opener when she fired across goal inside ten minutes and would play a huge part in breaking the deadlock, albeit in slightly fortuitous circumstances.
The winger whipped a dangerous corner into the Brighton near-post from the left, with Seagulls stopper Megan Walsh parrying the cross into the unfortunate Veatriki Sarri and over the line.
The blue touch paper had been lit, with Alex Greenwood’s looping free-kick headed just wide of the post by Bunny Shaw moments later.
But City would make the most of our dominance once again when Blakstad met Kelly’s left-footed cross to head home her third goal in as many matches before the 20-minute mark.
Gareth Taylor’s side were in control, but visiting midfielder Julia Zigiotti threatened to halve the deficit when she tried her luck from distance after intercepting a pass from Laia Aleixandri.
On this occasion the effort was too high to trouble the rapidly retreating Ellie Roebuck, and number three for the hosts would follow in the subsequent passage of play.
Another penetrating run through the middle from Blakstad was eventually cut short by the Brighton defence, with the ball ricocheting into the path of Coombs.
Presented with a clear sight of goal, the midfielder made no mistake, rifling into the top corner from the edge of the box for her first goal since the opening day.
Coombs almost added a spectacular fourth when she acrobatically volleyed back across goal on the stroke of half-time, but this time Walsh was equal to the effort, tipping it expertly away from danger.
The opening exchanges of the second 45 followed a similar vein to the first, with Castellanos hitting the outside of the post from a tight angle after a clever reverse pass from Kelly early on.
City's number nine continued to threaten down the flank, cutting the ball back into the path of substitute Filippa Angeldahl, but the Swede couldn't sort her feet out in time to get a clean contact on the shot.
But with the visitors sitting deep in two stubborn banks of four, it became increasingly difficult for Gareth Taylor's side to find another opening as the match ticked over the hour mark.
Castellanos went close with an instinctive first-time drive from Greenwood's cut back from the by-line, but Walsh was equal to the effort on this occasion.
Following that chance, Brighton began to slowly grow into the game, with their standout player Zigiotti's audacious effort from distance flying just wide of Roebuck's post.
City continued to press forward in search of further reward, with Blakstad looping an effort onto the roof of the net as stoppage time approached.
But as the game reached its dying embers, the visitors managed to grab a late consolation when Lee Geum-Min bundled the ball over the line after Roebuck had saved the initial effort.
Not that it'll bother Taylor nor his players, who celebrated a sixth WSL victory in succession to set up a captivating showdown in next Sunday's Manchester derby at the Etihad Stadium.
Player of the match: Yui Hasegawa
Another accomplished display in the heart of midfield, the Japanese international kept City ticking over across the 90 minutes.
Composed on the ball and tenacious without it, Hasegawa continues to impress after joining from West Ham on deadline day.
What it means
City remain fourth in the WSL, but victory moves us back to within three points of Manchester United and Arsenal.
Gareth Taylor’s side are currently six points off Chelsea, but the league leaders have played one more match.
How we lined up
Gareth Taylor made five changes from the side who beat Sunderland 3-0 in the Continental Cup, with Laia Aleixandri, Kerstin Casparij, Laura Coombs, Deyna Castellanos and Chloe Kelly all reintroduced.
Ellie Roebuck started between the sticks, shielded by a back four of Casparij, Aleixandri, Alex Greenwood and Leila Ouahabi.
Yui Hasegawa started at the base of a midfield three alongside Coombs and Castellanos, with Kelly and Julie Blakstad offering width for Bunny Shaw at the sharp end.
City XI: Roebuck, Casparij (Morgan 61'), Aleixandri, Greenwood, Ouahabi, Hasegawa, Castellanos (Mace 73'), Coombs (Angeldahl 46'), Kelly (Raso 61'), Shaw, Blakstad.
Subs: MacIver, Stokes, Houghton, Kennedy, Hutchings.
Brighton XI: Walsh, Fox, Pattison, Williams, Green, Zigiotti, Connolly, Lee, Carter, Sarri (Bance 85'), Robinson
Subs: Startup, Jarvis, Park, Stott, Symonds, Terland.
City travel to Prenton Park to face Liverpool in the Continental Cup on Wednesday 7 December.
Kick-off is scheduled for 19:00 (UK), with the match available to watch live on CITY+ and Recast.
Our next league fixture sees us lock horns with Manchester United on Sunday 11 December.
With the match taking place at the Etihad Stadium, and over 40,000 tickets already snapped up, it promises to be an historic occasion.
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