The name Togodumnus is mentioned twice in book sixty of Cassius Dio's History of Rome. Both passages are quoted below:
"... [Plautius] first defeated Caratacus and then Togodumnus,  the sons of Cynobellinus, who was dead.  ... After the flight of these kings he gained by capitulation a part of the Bodunni,  who were ruled by a tribe of the Catuellani;  and leaving a garrison there,  he advanced further and came to a river.  ..." (Dio lx, 20.1-2)
"Shortly afterwards Togodumnus perished, but the Britons, so far from yielding, united all the more firmly to avenge his death. ..." (Dio lx, 21.1)
There is no evidence of the existence of Togodumnus in classical literature other than the words of Dio (quoted above). Our other primary source of information regarding this period, the works of Cornelius Tacitus, are unfortunately incomplete, and amongst the missing chapters are those dealing with the invasion of Britain in 43AD.
Unlike his brother Caratacus, Togodumnus issued no known inscribed coinage, which is unusual; iron-age monarchs in Britain had begun to issue inscribed coins prior to the turn of the first millenium, primarily to let their tribes people know who was now in charge. To explain this anomalous behaviour, Graham Webster [TRIoB, pp74] suggested that in c.40AD when Togodumnus and Caratacus apparently took control of the Catuvellaunian state, they did so not because the old King had died, as had been supposed by most other historians, but because he had been near fatally injured or had suffered a stroke. Webster says further that old king may have been still alive during Caratacus' annexation of the Atrebatean territories, though incapacitated. Togodumnus had apparently stayed at Camulodunum as co-ruler because his fathers condition had so enfeebled him as to make his continued rule a physical impossibility. If this was so, for Togodumnus to have issued his own coins from Camulodunum whilst his father was still alive would have been the height of indiscretion. Caratacus however, would get away with issuing coinage of his own in the captured lands of the Atrebates.
Dio tells us in the passage quoted above, that Togodumnus and Caratacus were both sons of Cunobelin, thus giving us the name of his father and that of his brother. We know also of Adminius [Suetonius, Caligula, XLIV.2], another son of Cunobelin, who, therefore, must have been the brother of both Togodumnus and Caratacus; or a half-brother at least.
We do not know the name of his mother (although we can be certain that he was blessed with one), for Roman historians in general dismissed the womenfolk as un-noteworthy, even though in a true iron-age society such as existed in Britain at that time, it was perfectly acceptable for a tribe to be ruled by a female monarch, Boudicca and Cartimandua being cases in point.
We can account for three sons of Cunobelin who are mentioned in Dio, Suetonius and Tacitus;
As pointed out by Webster [Caratacus, fn.2 p.152], based on the evidence of Tacitus [Annals XII.xxxv], following the final defeat of the resistant British forces in Wales, the wife, daughters and brothers of Caratacus surrendered. Since the plural brothers was used by Tacitus, indicating that at least two persons had surrendered, it follows therefore, that the old king of the Catuvellauni had at least five sons.
The entire argument fails if Tacitus' statement is read with different emphasis, for example, if brothers were meant more in a 'comrades-in-arms' sense, though this is unlikely. Another interpretation is that Tacitus' statement should be construed to mean, or at least to include, the 'brothers-in-law' of Caratacus. The fact that Dio mentions only Caratacus by name may also be of significance. Although Dio is sometimes painfully brief, it is likely that he would have recorded the names of any legitimate sons of Cunobelin that surrendered themselves in battle, had they been known.
In light of all these arguments, I am inclined to believe that Cunobelin had only the three sons mentioned in the classics, and that the 'brothers' of Caratacus who surrendered to Ostorius Scapula in the account of Tacitus were indeed, his 'brothers-in-law'. If we accept this, then we must also conclude that Cunobelin had at least two daughters, thereby, sisters of Togodumnus.
We are reasonably certain that Togodumnus was older than his brother Caratacus, as it was he who took the vacant Catuvellaunian throne following their father's death, whereas Caratacus was prompted to seek territory for himself in the lands of the Atrebates.
In the debate as to who was the eldest son of Cunobelin, many modern historians have favoured Adminius, using the argument that it was he who was given jurisdiction of Cantium by Cunobelin in c.35AD. If we accept the scenario in which the eldest son is Togodumnus, then Adminius' kingship of Cantium may be explained by the following outline;
It is very likely that the rule of Cantium would have been offered to the eldest son of king Cunobelin. If Togodumnus were the eldest, it would appear that he passed-over the kingship in favour of the next in line, namely his younger brother Adminius. This may have been done outwardly as a favour to his younger brother, but in reality it was to place him well out of the way, and remove him from political affairs in Camulodunum, where his pro-Roman tendancies would be a hindrance to the plans of his two brothers, Caratacus and Togodumnus. Adminius, therefore, given the chance of the kingship of Cantium in such an act of filial piety by his elder brother, could not have reasonably refused the offer. Togodumnus had perhaps pleaded that he himself be given rule of the Catuvellaunian heartlands from the old tribal capital Verulamium, in order not to appear ungrateful for refusing Cantium. This seems to have been the case, for it appears that Verulamium underwent a period of increased trading activity at this time, perhaps because a scion of the old ruling house was again in residence.
We are not told whether Togodumnus was married, though we can reasonably assume that he did have a wife, as marriage was an integral part of the iron-age noble tradition. It is likely that the wife of Togodumnus, accepting that she had once in fact existed, was dead before the Roman invasion of 43AD, as no mention is made in Dio's narration of the events. It is possible that a wife may have survived him, but as Tacitus' later account of the surrender of the Wife and 'Brothers' of Caratacus fails to mention her, we may only conclude that Togodumnus' wife, had she existed, must have died sometime before 50AD.
Dio and Tacitus also omit to inform us whether Togodumnus had any children. Admittedly, this could be for several reasons;
The last example is very unlikely, for any son of Togodumnus would have been heir to the throne of the Catuvellauni tribe and therefore extremely noteworthy. I am inclined to believe that either Togodumnus did not have any offspring, or that like his brother Caratacus, he had all daughters.
The Catuvellaunian kingdom had been ruled from Camulodunon only since c.9AD when Cunobelin wrested the city from the rival Trinovantes tribe in the wake of the Varus disaster in the Teutoberger forest of Germany. It is very probable that Togodumnus, being arguably the eldest son of Cunobelin, was born prior to this date. It is fairly certain, therefore, that Togodumnus was born at Verulamium (St. Albans), the previous Catuvellaunian capital.
We are reasonably certain that when Togodumnus' father Cunobelin died, in c.40-43AD, he was an old man of between sixty to sixty-five years of age; he would have been born sometime around 25BC. Within iron-age society it was usual for a young noble to marry at a fairly early age, certainly before his late twenties, primarily because the life-expectancy of a British Celt at the beginning of the first millennium was only around forty-five to fifty years. Given this, we may reasonably assume that by the age of about twenty-five, Cunobelin was both married and was already a father. It follows, therefore, that Togodumnus could have been born at the turn of the first millenium, and at the time of his death in the wake of the Roman invasion, he was probably aged around forty.
As mentioned above, we are informed by Suetonius that a son of Cunobelin named Adminius, was given rule over the east of Kent. This was an important assignment, as the region included the only land-locked harbour on the south-east coast of Britain, and also the strategically important Wantsum Channel into the Thames Estuary.
Following Cunobelin's enfeeblement [c.40AD] Togodumnus took over the rule of the Catuvellauni from Camulodunon, and Caratacus began to invade the lands south of the Thames [the Atrebates and the Belgic tribes later to become known as the Regni].
That Togodumnus and his brother Caratacus were strongly anti-Roman is evident enough by their opposition to Plautius advance through Kent and at the Medway. This resentment seemed to have surfaced earlier [in 40AD] when their other brother Adminius was expelled from Britain by the old king, supposedly for his pro-Roman opinions, which supposition is strengthened by his immediately seeking an audience with the mad Roman emperor Gaius 'Caligula', who was then on campaign in Germany.
Whether the two remaining brothers had coerced their father into banishing Adminius is uncertain. What is certain is that as soon as their father had died, another pro-Roman British king, the elderly statesman Verica of the Atrebates, was forced from his remaining lands in the south of Britain, later to appear in Rome before the Emperor Claudius. This was to prove a fatal mistake, for Claudius the new emperor was looking to make his reputation as a general, and siezed on the appearance of Verica as an excuse to wage war on Britain.
The death of Cunobelin in c.42AD resulted in a rapid change in polarity in the Catuvellaunian court, the elder statesmen of the tribe, former aides to the old king were dismissed, to be replaced by the retinue of the two anti-Roman princes. Adminius in Durovernon, cut off as he was from events at Camulodunum, would nontheless have had his nose in the air regarding any changes in the political wind. Thus he learned of the death of his father in time to take evasive action, and sensing his life was in danger, fled from Cantium to Gaul.
Webster [TRIoB, pp73] emphasises the rapidity with which the political situation was changed following the death of Cunobelin, which showed that Togodumnus and Caratacus had spent some considerable thought on their joint strategy, and had been waiting for some time for the event to occur. That brother Adminius was not privy to these plans is proved by his rapid expulsion from Britain.
That the druids condoned the actions of the two Catuvellaunian princes is without dispute, for it was the druids, whose ranks were drawn from the cream of the British nobility, who apart from holding the monopoly on all things intellectual, also had control of the state religion of the island, and were able through their vast network of inter-tribal connections to "split tribal loyalties", "organize palace revolts", and "to secure thrones for their proteges". Togodumnus and Caratacus could hardly have acted without the druidical orders approval, therefore it must be concluded that they had secured the backing of, and were probably acting in concert with, the druids.
To intercept the Roman advance, Togodumnus presumably had to travel from his capital seat at Camulodunum, some forty to fifty miles to the crossing of the Thames, then get his forces over the river and proceed a further twenty or thirty miles into Cantium.
Caratacus was possibly installed at Calleva, the captured capital of the Atrebatean kingdom south of the Thames, about seventy miles away from Durobrivae on the Medway, along the course of the South Downs Way. With no major rivers to cross apart from the Medway itself, Caratacus, if based at Calleva could have reached the advancing Roman army well before any forces from Camulodunum under Togodumnus.
If we are to believe Dio and accept that the "Bodunni" came to terms with Plautius following the defeat of Caratacus and Togodumnus in Cantium prior to the Medway, then we must believe that they did so in the narrow period between the defeat of the second force of British chariots under the command of Togodumnus, and the Romans reaching the Medway. This could have been a period as short as a single day, given that the garrison mentioned in Dio's text had been left at Durovernum [Canterbury], only twenty five miles to the east.
The Dobunnic territories lie in Gloucestershire, some fifty miles beyond Calleva, further to the west, and to reach Cantium, the most obvious route would be south-east to Calleva, then east along the SDW. For the Dobunnic leaders to have capitulated to Plautius prior to the Medway battle, they must have followed directly in the wake of Caratacus' army departing Calleva. They would also have met with and possibly impeded the retreating armies of Caratacus and Togodumnus as they fled west along the SDW to escape the Romans.
It is my thought therefore, that Dio got his facts right, but in the wrong order, and that the Bodunni came to terms with Plautius following Battle of the Medway and more importantly, the death of Togodumnus.
The Dobunni, being a dependant state, would have been required to swear alliegence to the legitimate heir of the Catuvellauni following the death of king Cunobelin. This would have been the Catuvellaunian prince Togodumnus.
Since he had shown himself to be the more diplomatic of the two brothers, and also being the elder, he had probably made representation to the Dobunnic court on behalf of his father on a number of occasions in the past prior to the old kings death.
When the Dobunni were asked to pay respect to the new regime in Camulodunon, this was probably given freely at first, but later, when the anti-Roman tendancies of the two brothers became almost obsessive, they evidently regretted their decision, being an essentially pro-Roman state. So that following the death of Togodumnus, they no-longer felt obliged to support Caratacus, as they had had no previous diplomatic communications with him and were probably glad of the excuse to dissolve the old alliance and seek the protection of Rome.
Togodumnus was probably in overall command of the combined British tribal army at the crucial Medway battle; his brother Caratacus presumably his 2IC.
There were very heavy losses to the British contingent at the Medway, and Togodumnus died of his wounds after the battle [possibly within a few hours, certainly within one or two days]. Caratacus then took command of the British armies and whipped them into a frenzy, making them all the more determined to resist and avenge the death of their war-lord.
"Although Togodumnus and Caratacus raised a better organised and better armed force than any of the other British leaders, once their army was beaten the war was over." (Rivet, TaCiRB, p.53)