From the view passengers see of this station, it all appears pretty straight forward, but it is of course where the District and Hammersmith lines converge and diverge to and from Tower Hill and Liverpool Street respectively.
The area to the west of the station is known as Minories Junction; this forms a flat junction, with Aldgate station located in its centre, Aldgate East (as the name suggests) to the east, Liverpool Street to the north and Tower Hill to the west. It is the only area where the signalling is not controlled by the District Line - signalling is done by the Metropolitan Line Control Centre at Baker Street. Control is transferred as a District Line train departs eastbound from Tower Hill and is returned to the District Line as a train approaches Whitechapel.
This station was opened in 1938, replacing the original station of the same name which was located some 200 feet to the west of its current location. The original station was not only small, but its location also caused delays to trains in the area as a full length train could not stand in the platforms without fouling the junctions at either end. More about this can be read in Piers Connor's book 'Going Green'.
The following views are intended to give a general 'feel' of the location, rather than of the specific signals involved, though I'll give some detail of these and of the possible 'moves' as appropriate.
The first two photos were taken from the westbound platform.
The station platforms are a little unusual as there are exits from both ends. This view is looking east. From the westbound platform it is possible to reverse a train back towards Whitechapel.
This is the view looking westbound. The Station Starter is just visible between the east and westbound tracks (known as the 6 foot - being the approximate distance between the two lines) and also has a Junction Route Indicator associated to it. The Hammersmith & City line expects a route to the right (the Junction Indicator illuminates for this), though in the event of problems in the area from Liverpool Street can be diverted via Tower Hill and thus round the 'outer rail' of the Circle Line. District trains expect the route to the left (a plain green signal) and cannot go via Liverpool Street.
Taken from the concourse at the west end of the station, a C Stock waits to head east.
Update added 21 July 2004
Solidbond has provided some more detailed photos which relate to the rebuilding of the station which I refer to above.
During the reconstruction the tracks to be lowered and the originals had to be suspended to allow the work to take place below. The eyes from which this was done are still visible and can examples can be seen in this picture.
Closer detail of the eyes used as described above.
As you travel between Tower Hill and Aldgate these pillars are visible; they formed part of the original station structure.