One NPC against multiple PCS - tactics

[LucyG92]

A party of four is soon going to fight one guard (a Barbarian). The guard knows the area - one floor of a tower - and it's likely he'll have a few minutes to prepare. I want this to be at least a little challenging, and without just adding class levels. What kind of tactics can I use? There aren't really any chokepoints he can use.

[Wheldrake]

It's a tower... what about stairs? Throwing furniture down the stairs could slow or injure his attackers. What about doors? A door can be a good choke point, and the barbarian might even be able to slam it in their faces, like a bull rush, which could be even better if there are stairs down behind it.

Any traps possible? Rigged crossbows? Heavy stones ready to fall? Pet attack dog to help even the action economy?

Basically anything to keep one or more attackers busy while the barbarian deals with one guy at a time.

[Sissyl]

Tanglefoot bags. Thunderstones. A potion of invisibility usually works well to get at squishies. A silenced axe.

[tonyz]

Flipping over a table to fight from behind cover.

Spreading some oil (or other liquid) on the floor to make it slippery. If it's flammable liquid then throwing some alchemist's fire would make some nice burning stuff for the PCs to deal with.

Throwing a blanket or something over someone's head to blind them.

Setting something on fire to create smoke to block enemy vision and maybe make a room hard to enter.

Setting up a chair or sofa against a door so it falls against the PCs if they open the door (bull rush some back, make the doorway difficult terrain)

SOUND THE ALARM!!! More guards should be coming soon.

[Gavmania]

As a Barbarian, he's presumably a melee monster. Take advantage of a target rich environment by setting off a horn of fog or eversmoking bottle. Anything the Barbarian meets in the gloom, he can attack, pcs will have to check whether its friend or foe first.

[GM and the Holograms]

I'll second the door and fog.
The door is both a choke point and cover if used properly. Of course whether he would do that while raging or not would take some justification, since it takes patience and intelligence. The best way to handle that I think would be to lay in wait and rage when they get there, you wouldn't have to move.
The fog would be the same deal, set it off before they arrive then rage when you run into one.
I think your big problem with challenging the players will be longevity of your barbarian. Depending how you build it the AC will be really low and soaking damage isn't a great long term solution against multiple foes.