Minecraft

I think you can get half of it by setting the difficulty to hard. This allows zombies break down wooden doors for example.

I can recommend The Late Pack if you don't mind a 1.12 pack.

During the night, mobs attack your base and can temporarily break blocks of your base to get to you (blocks reappear in the morning I think).
You have to build walls and turrets to defend yourself. Sleep is disabled.

As long as you don't try to cheese the pack by having floating houses (not sure about underground), it's a pretty good experience.

It only covers the base building part, but Mine Colonies is a mod pack with very extensive systems. Build a town hall to attract colonists. Build hotels or houses to give them a place to stay. Build a Builder's hut and assign a worker. They will build for you based on various themed templates. Your job is to manage job queues to fix bottlenecks, plan new construction, and go out into the Minecraft world to gather the increasingly rare materials you need to build the builder's hut lvl 2 or whatever.

I had a GREAT time with it, playing under ground using the Cavern building templates. https://ladyshatana.wixsite.com/mcguides/cavern-megahalls

I'm sure you could combine this with more difficulty mods. Your colony has guards if you build guard huts and archers if you build the right structure. Careful that too many of them don't die! Their friends and families will not want to work if they are grieving.

I just need to emphasize that 4GB of RAM for a minecraft server is really low - you're going to encounter a lot of stuttering and desync with more than 2, maybe 3 people unless you're all mostly sticking together. This is especially true with mods

I don’t really have anything off the top of my head except Lycanites (though I personally find it annoying). Still you could mine RLCraft and Gregtech New Horizons or other mod packs and see if they have mob mods you like

Valkyrien Skies. Then you can bring your base with you.

Not in a million years on that server though.

In all seriousness, second the guy who said Minecolonies. It turns Minecraft into SimCity. I really enjoy combining it with the Lost Cities. I like reclaiming ruined cities and repopulating them.

Years ago, I played on a server that had a bunch of mods that added more hostile mobs to the point that you had until the first night to build some kind of a basic shelter and then you just couldn't leave until you had full diamond armor and weapons (even then it was iffy). There were like goblins and ogres and tigers and bears and stuff wandering around outside. The chat was just " was killed by " for days. It wasn't very fun though.

If you really like base building and dealing with "mobs," you guys might like "The Forest" more.

I have a desktop running debian with 4gb of ram allocated to the server, so it needs to be kind of lightweight.

As others have said 4GB ram may be light for heavily modded Minecraft server.

I have gotten away with it, but only for about 4 players at a time and only with early game versions, and by choosing my mods very carefully.

Not to change the subject, but I would want to know so I'll mention Luanti.

I run heavily modded Luanti alongside reasonably modded Minecraft on a server with 8GB RAM.

I launch Minecraft with a limit that prevents it from taking more than 7GB, ensuring that Luanti always has 1GB ram to run in.

But, I am not aware of a good Luanti mod set for the kind of play you are looking to setup.

So guess, I would just keep Luanti in mind, if RAM becomes a chokepoint.