Just to give folk advice -- avoid hiring Gnarri.  Once a champion joins you, you can't get rid of them, and of all the champions in the game, Gnarri causes the most problems in terms of several subsequent champions refusing to join you because he's in your party (and, again, he can't be killed and you cannot remove him nor any other champion from your Sovereign's army).

Also ... unlike the sandbox game, there is really no point to investing in non-Champions in the campaign ... don't bother training military units nor summoning elementals nor the Familiar.  At the beginning, okay, the military units that join you are fine, but they will not go with you to the next map in the campaign, you can't move champions from your stack to escort around trained units, joined-you units nor summoned units, so the experience you try to gain grinding non-champion units through battle gets lost.  Currently there is a bug that I presume will be quickly fixed in the next patch in that with 9 or more units in your stack (despite a much smaller number of open slots appearing, champions and units continue joining your party even past filling the 3x3 grid; summoned creatures also add on), one or more will not appear in tactical combat (which is intentional, though perhaps intended to allow 9 in tactical combat instead of the current 8) but will be regarded as having been killed in combat at the end of combat (which is not intentional and is a bug) and will incur injuries as a result in addition to having their health eliminated.  Even when this bug is fixed, presuming they stay with an 8- or 9-unit limit in tactical combat, I still recommend removing non-champions from your stack (you can remove non-champion units normally) because the campaign continues across several maps, but only your champions will go with you (so you have to start all over training military units and gaining experience with them).

Your champions (with one exception) do carry over with their experience, as does your research, gildar, promotions for champions and thus spells, etc.  Build what buildings you need for guildar and especially research, and once all buildings are built, I recommend letting your city idle for research.