As a note, this is not my guide. I found it on LJ, and the wonderful omphalos is the one who created the guide. I've only adapted it for use in the RPG parabolical. All the hard work belongs to omphalos. Original guide can be found here.
Exteriors & Access
The Torchwood 3 Headquarters, otherwise known as 'the Hub', are located
under Roald Dahl Plass in Cardiff. The Hub can be accessed via a
special 'invisible' lift that emerges in front of the fountain; it's
protected from public view by a 'perception filter'.
The more usual access method is via the rundown Tourist Information
shack on the sea front. It's not at all clear where the SUV is parked,
but there may well be another access point via its garage. The team
park their personal vehicles (if Gwen's example is representative)
somewhere in the rough area I've marked.
Roald Dahl Plass
The Tourist Information Shack
The Back Way
It's unclear how deep underground the Hub reaches. While the central
Hub is a huge cavern-like space that includes at least two balcony
levels above the floor level, and at least one basement level below it,
there's strong implication of a lot more. Ianto mentions nine levels of
cold storage lockers, for instance. I have a hunch that the area I've
marked with a question-mark leads to a underground parking area for the
SUV, but I've no proof to back this suggestion at all.
Key: A - TOSH'S DESK. B - OWEN'S DESK. C - WAY DOWN TO INTERROGATION ROOM. D - GAMING AREA. E - AUTOPSY ROOM. F - JACK'S OFFICE. G - SOFA. H - DISPLAY AREA. I - GWEN'S DESK. J - MAIN DOORS. K - FOUNTAIN L - WORKSHOP AREA. M - ARMOURY/BOARDROOM. N - BALCONIES. O - STAIRS DOWN TO CELLS. P - STAIRS UP TO BALCONY. ? - MYSTERY AREA.
Main Doors (J)
The Fountain (K)
Workspace Platform (A, B, G)
The Autopsy Room (E)
Jack's Office (C, D, F, H)
The Armoury (M)
Gwen's Area (I, L)
The Balcony (N, P)
The Boardroom (M)
Other Hub Locales
It's not completely clear where some of the locations below are
situated in the Hub. The stairs down to the cells seem to be located to
the left of the armoury, which I think is also the route to Lisa's
room. Access to the cells themselves seems to require climbing down a
metal ladder at the end, however.
I've no clue where the cold storage locker rooms are. It says 'cold
storage' above the arch that leads to the autopsy room, but that seems
to lead nowhere but the autopsy room, which does have limited cold
storage space. Judging by the place from where Gwen and Jack emerge at
the end of End of Days, cold storage may well be accessed via the basement stairs used also for the cells and Lisa's room.
The interrogation room is accessed via a short staircase down found
outside Jack's office. The shooting range location is anyone's guess. ;)
The Interrogation Room
Cold Storage Lockers
The Shooting Range
Other people's guides:
Official BBC (includes an animated guide to the hub)
Torchwood Locations (shows above ground filming locations around Cardiff)