The East African army at the Songgo military base watched as the Portuguese busied themselves outside the city, while the Mozambique army was constrained by the Black Corps, possibly because the French had taught them how to make siege equipment during training!
Africans wouldn't be incapable of making ladders, but this craftsmanship depends on the region. For example, the indigenous people in the tropical rainforest regions build nests in trees, so wooden ladders are essential tools. Some more developed indigenous kingdoms also have similar needs and can make simple wooden ladders.
