We are indeed indebted to Richard Tennant’s articles on cartography on the Napoleon-Series for identifying this map (see below).
It is actually possible to view the entirety of the road network and terrain between Pombal and Almeida. Why via Pombal and why take the route south of the Mondego? Pombal is the staging post north of choice, simply because of the difficulties of crossing the mountainous terrain to the east of Pombal – the foothills of the Serra de Estrella. In terms of the route south of the Mondego, this was Wellington’s most practical route north to Almeida. Of course it was entirely possible to travel north from Castello Branco, in Beira Baixa, along what it referred to as the “Estrada Nova” northwards – but this was a most difficult route and was hardly practicable for wheeled vehicles. And, more importantly, this route was even more inland, than the route via Pombal, and Wellington could not benefit from the Royal Navy’s domination of the sea and logistical support from the navy via the Mondego (and Tejo) – thanks, in great part, to the extremely positive relationship Wellington enjoyed with Berkeley. Of course in the third invasion of Portugal, Massena chose routes to the north of the Mondego. We have discussed the reasons for this many times previously, but essentially this decision was based on inferior military intelligence and logistical expertise.

https://digitalarchive.mcmaster.ca/islandora/object/macrepo%3A78179
A