I managed to soft lock the game, basically I was in the process of recruiting a new unit in a city, it had one population, then the "loss of population" event coincided with the recruitment of the unit, as a result the city had a population of -1 I couldn't select anything to build as the city had "not enough population" to build or do anything...but to end the turn I needed to select something to build there...so the only option I had was to concede defeat...