This scenario is pretty basic and doesn't have many triggers but it is fun and original and has a believable story. The story is set during the Roman occupation of England and tells of clans of Celts joining in a union to protcet their homeland.
The scenario begins with you being the Tayside clan and having your capitol at Dundee.
Wikipedia says, "Tayside (Scottish Gaelic: Taobh Tatha) was one of the nine regions used for local government in Scotland from 15 May 1975 to 31 March 1996. The region was named for the River Tay."
But it doesn't seem to have been a name for a clan.
Dundee is on the side of the river Tay in Tayside.
Dundee has a hill called Dundee Law. Wikipedia says, "During the Iron Age it was the site of a Pictish settlement. Roman pottery has been found on the law, suggesting that the Romans may have used it as a lookout post in the first century."
Wikipedia's article on the Romans in Scotland says, "Roman legions arrived in the territory of modern Scotland around AD 71, having conquered the Celtic Britons of southern Great Britain over the preceding three decades. Aiming to complete the Roman conquest of Britain, the Roman armies under Q. Petilius Cerialis and Gn. Julius Agricola campaigned against the Caledonians in the 70s and 80s."
The Romans abandoned the Antonine wall north of Glasgow in 165 AD, retreating to Hadrian's wall in northern England near Carlisle.
In the first scenario, there are Roman cavalry near your city.
In the first scenario, Hadrian's wall has been built, and the wall was begun in 122 AD. Your goal is to fight the Romans because they are planning to occupy Scotland. They didn't seriously plan on occupying Scotland though after their 165 AD withdrawal to Hadrian's Wall.
The second scenario has you attacking a convoy north of Hadrian's wall going to supply Rome's more northern forts. The implication is that Hadrian's wall is in use but the Antonine Wall is not. The Antonine wall was built in AD 142. This implies that the mission takes place in 122-142 after Hadrian's Wall was built but before the Antonine Wall was built, although conceivably it takes place after the Romans left the Antonine Wall. They left Roman Britain in 410 AD. The Romans stopped campaigning in Scotland after Emperor Caracalla's death in 217.