With Cities: Skylines, players have the tools and freedom to create the metropolis of their dreams, whether it’s one that hums like a finely tuned machine or one that puts more emphasis on aesthetics than function. What helps the game stand out from other city builders is its extensive modding community, which creates and shares game additions that range from new buildings to literally rewriting some of the rules.

Developed by the Finnish studio, Colossal Order, the simulation game remains as one of Paradox Interactive’s top-selling titles. Earlier this month, the game hit its second anniversary and the publisher announced that it had sold 3.5 million copies worldwide. They celebrated the milestone by giving all its players a special set of in-game buildings inspired by Chinese architecture.

“From the first day Cities: Skylines was released, it’s been breaking records,” said Fredrik Wester, CEO of Paradox Interactive, in a statement. “The game had our biggest launch in history at the time, selling 250,000 copies in its first 24 hours, and the community has only gotten bigger and more devoted over the last two years.”

Now the city builder is ready to grow even further with a new expansion called Mass Transit. As its title implies, the premium expansion focuses on public transportation, which Colossal Order is very familiar with, having released a mass transit simulator called Cities in Motion in the past. The developers wanted to get back to its roots with its latest expansion, and transportation is huge part both a city’s infrastructure and the city building experience.

Mariina Hallikainen, CEO at Colossal Order, sat down to speak with [a]listdaily about the upcoming expansion, releasing premium content for a game that’s famous for its mod content, and engaging with its active community.

What inspired the new expansion and what does it include?

Cities: Skylines – Mass Transit was very much inspired by our Cities in Motion games, which was all about mass transit. As an expansion, it’s all about creating good public transportation, as it is a big part of any city. It brings four different types of transportation: ferries, blimps, elevated monorails and cable cars.

Cities is very connected with its mod community. How do you develop a game expansion with modders in mind?

We, as developers, always have a certain vision for the game and we need to maintain a certain amount of control over that vision. We love talking with our community and taking their feedback, but we also must stay true to ourselves. It’s really a collaboration between the modders, us and the community.

For example, ferries is something that people have been asking for, and we agree that the game needs that transportation option. In that case, it’s very easy to work that in. Of course, sometimes we disagree with the community, but I think that’s a healthy conversation that needs to be had. I think that as long as we keep talking to each other we’ll be fine. [It’s about] taking the feedback and being honest with them. If we don’t like something that they suggest I say it honestly: this is not going to happen. We’re not going to do it. Then there will be a little bit of griping, and hopefully we’re able to move on to the next suggestion.

With so many free mod options available, how does the Cities audience generally take to having to buy premium expansions?

We want to make expansions that are worthwhile. I think mods enhance the game in amazing ways. The things that the modders do—they go all out. We have over 100,000 items on the Steam Workshop, so there’s a lot of variety.

For us, it’s important that we can add features that modders can’t easily do. [Also], not all of our players use mods. We always have to remember that the modding community is very active and open, but they are not the majority of players. So, it’s about finding a balance between the different audiences and trying to cater to all of them. The important thing about the paid expansion is that we know it’s not necessary for everyone. So, we want to do our best to bring in new and interesting content that will appeal to a large portion of our audience.

How do you balance between releasing free updates and premium content?

Free updates are features that we believe enhance the base game in such a way that it would be unfair for us to charge for them. We want to keep working on the game to make the experience good for existing players who have the base game, and those who want to buy more content may get the expansions. For example, the road enhancement tools were something that we wanted all the players to have, so we definitely weren’t going to put that behind a paywall.

How do you engage with players who aren’t part of the modding community?

We have an amazing subreddit. I think there are over 100,000 subscribers on the Cities: Skylines subreddit and it’s very active. There are so many people creating beautiful cities and sharing screenshots and videos. There’s a lot of feedback to be seen over there, so I try to be very active there. Then we have the Paradox forums where we have an amazing community. We feel that we need to be active through social feeds: Twitter, Facebook and so on, to make sure that we find different kinds of audiences. The modding community can be the loudest and most active, but we want to stay in touch with players who aren’t necessarily engaging with that part of the game.

What goes into the perfect city building game?

I think the perfect city building game has the flexibility for players to make their dreams cities. It lets their players use their creativity. I think that we’re almost working on a platform for the players to express themselves with. Of course, there also needs to be functioning systems that makes sense for the player. It’s a combination of those systems, freedom and flexibility.

Cities: Skylines remains one of Paradox Interactive’s top games. What do you think attracts so many people to city building games when there are so many explosive action games around?

I think city builders in general have a very wide range of people who like to play them. There are gamers who just who want to take time off from everything blowing up and just relaxing with a city builder to create something. But I think there is also an audience that doesn’t play other kinds of games. They are just engaging with simulation games because they feel that they can express themselves and solve problems without the fast pace that other genres offer. It’s an interesting crowd, to say the least, because there are so many different types of players. To see the different things that people do with Cities: Skylines—some just want to build a beautiful city, while there are other people who want to make sure the city is functioning in a perfect way without caring about the looks of it.

What would your perfect city be like?

One that makes a lot of money [laughs]. I’m a CEO, after all. So basically, that’s all I care about.