Change is inevitable, especially in the music world, and especially in Bend, where the population rises every year. Look to the city’s annual music festivals for proof — events such as 4 Peaks Music Festival, Sisters Folk Festival and the myriad street festivals downtown add new features, expand lineups, shift to more (or fewer) days, move venues or otherwise evolve year-to-year.

Not so much at Central Oregon Metalfest, which returns to Third Street Pub for its sixth year Friday and Saturday. When GO! Magazine asked Coma Booking and Promotions owner Mike Self what’s new for this year’s festival, he kept it succinct: “More metal.”

“(There’s) nothing really new other than just more variety of bands and bands from further away,” he said. “… I think this year, the band Mortal Ashes from Colorado, they’re definitely the ones traveling the furthest.”

In other words, Central Oregon Metalfest is all about the music. For Self, also a longtime metal musician who has played with various local bands, the event is a boost for the underground metal scene not just in Bend, but throughout the Pacific Northwest and beyond.

“The thing to keep in mind with this genre of music: There’s a lot of value in the underground-type thing,” Self said. “These bands on average are playing their hometowns to an average crowd of 20 to 30 people. That’s just average all the way across the board. And these bands come up here and they find out that they’re playing to a full house, and they look out and they get to see the crowd moving. It’s kind of uplifting for them.”

Self, who books frequent metal shows at Third Street through Coma, said the festival has averaged crowds of about 75 to 80 people Fridays and about 150 people Saturdays in past years, and he’s expecting about the same this year. The festival’s most successful year was in 2016, when supergroup Act of Defiance (featuring former members of Megadeth) headlined with San Francisco classic/doom metal trio Castle.

“We’re still guaranteed a draw of around 150 people even if each band only brings five people,” Self said. “It works out that way, and I think that’s what the bands are talking about. They say they have a good time; it was great. With the amount of people they get to play to — I think that right there is what keeps the bands coming back. That, and there’s no egos with anybody, with any of the bands that I’ve worked with, especially with the festivals. Everybody’s there for the music, and everybody seems to have a good time.”

Castle is once again on the 29-act bill this year, performing Friday evening. The band’s first Bend show was in 2014, said vocalist/bassist Liz Blackwell.

“We really love the underground metal scene, and especially places like Bend that has a good scene and community,” Blackwell said. “The festival that (Self) puts on is really great, and its spirit and atmosphere that can create this community and reinforce it. Underground metal works in a family style like that — you have to reiterate the connection with friends.”

If Blackwell seemed a bit distracted during her brief conversation with GO! Magazine, it was for good reason: She and her bandmates — guitarist Mat Davis and drummer Al McCartney — were on their way to The Hallowed Halls studio in Portland to put finishing touches on Castle’s fifth studio album. The trio once again worked with Billy Anderson (Sleep, Eyehategod), who produced the band’s previous three albums including 2016’s “Welcome to the Graveyard.”

Since forming in 2009, Castle has toured relentlessly throughout North America and Europe (the band is signed to German label Ván Records). Its heavy-yet-melodic sound draws inspiration from the sludge and doom genres as well as classic metal bands such as Judas Priest.

The project grew out of songs Davis wrote on his own in the mid-2000s. Blackwell joined forces with the guitarist in 2009.

“We push each other to get the sound that we want,” she said. “We’ve kind of been working on creating this partnership to build these albums. It’s cool. It’s like a hive mind kind of thing between us two.”