• Outpost limited spawn with the ability to recycle or renew (reduce max TTL each time) or even heal inefficiently.
• Palisade Weak walls that block movement or have huge fatigue for all units and decay like ramparts.
• Sentry Range 5 tower-like structure that can attack
• Watchtower Provides vision of adjacent rooms at the cost of energy per tick.

The idea of combat engineers that create things on the battlefield or setting up skirmishing outposts now that we have power creep harassment seems interesting. With the addition of the mysterious factories, and the pull ability it would be cool to have special buildings that you can drag into place.

Also, siege specific non-creeps that you bring with you would be very interesting. Things like

• Siege Towers Lets you bypass a wall, needs to be defended.
• Battering Rams Let units not adjacent to wall use their work parts to damage wall.
• Forcefield Reduce incoming ranged damage in a small region with a constant energy upkeep.
• Undermine Able to slowly construct a tunnel through a wall, let you bypass incredibly high strength walls if you can successfully complete it.

One of the most complicated parts of maintaining sieges in real life is the supply chains. Making it so that there are ways to attack that involve something more complicated logistically than sending streams of units to slowly damage walls could be interesting.

I don't think Siege weapons work with the way that rooms work in this game. If you were guarenteed more space between the edge of the room and the wall it could work, but if they're actually useful people will just build walls around the edges and make them very hard to deploy.

The ability to renew creeps and vision is IMO worth the work.
It should have a really rapid decay (5E sec?) so that maintenance cost is high and maybe 1000 shared res storage.

Once you have a Outpost you should be able to build Palisades in that room.