.

I found some references for what I need, from collections of historical costumes and open archaeology reconstruction sites.

The Freilichtmuseum Treppenhäuser features reconstructed late medieval German buildings (late medieval is ideal, since it is more or less passable for the entire period from the migration period to early modern times), and some of them could serve as a reference for the game's buildings:

Bergmeister's House (could serve as a reference for the teuton Town Hall?):
http://www.bergstadt-bleiberg.de/galerie/index.php?kat=bergmeisterhaus

Blacksmith:
http://www.bergstadt-bleiberg.de/index_02.html?/haus11/index.html
http://www.bergstadt-bleiberg.de/galerie/index.php?kat=schmiede_neu

Stables:
http://www.bergstadt-bleiberg.de/index_02.html?/haus17/index.html

There's also a reconstructed medieval house in Nienover which could serve as general architectural style reference:
http://openarchaeology.info/venues/mittelalterhaus-nienover-de

This small stronghold for the lower nobility could also serve for general architectural style reference:
http://openarchaeology.info/institutional-members/bachritterburg-kanzach-de

The book Costumes of all nations : 123 plates, containing over 1500 coloured costume pictures by the first Munich artists is available digitized online and contains a picture of a Germanic warrior of the migration period (for the unit it is not a problem if it looks very specific to the migration period, since it should be upgradeable later on to a more advanced type):
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=717634&imageID=810216&word=Germanic%20peoples&s=3&notword=&d=&c=&f=2&k=1&lWord=&lField=&sScope=&sLevel=&sLabel=&sort=&total=12&num=0&imgs=20&pNum=&pos=11

The book Historia universal : tomo decimosexto, segunda de la historia del traje also contains a few valuable references, such as one for the clothing and arms of Germanic peoples:
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=700083&imageID=817288&total=12&num=0&word=Germanic%20peoples&s=3&notword=&d=&c=&f=2&k=1&lWord=&lField=&sScope=&sLevel=&sLabel=&sort=&imgs=20&pos=12&e=w

From that same source, Longobards and Franks in the times of the Merovingians:
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=717598&imageID=810224&total=186&num=140&r=14f14570&word=&rOper=2&stype=Rel&rSource=&rDiv=Picture%20Collection&rCol=The%2520Picture%2520Collection%2520of%2520the%2520New%2520York%2520P...&s=3&notword=&d=&c=&f=&k=1&imgs=20&pos=155&e=w

From the The Vinkhuijzen collection of military uniforms, there are a few pictures of Frankish warriors:
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588356&imageID=1235258&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=2&snum=&e=w
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588373&imageID=1235275&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=19&snum=&e=w
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588376&imageID=1235278&total=33&num=20&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=22&snum=&e=w

From that same collection, I found a few references for the Celtic warrior:
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588362&imageID=1235264&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=8&snum=&e=w
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588364&imageID=1235266&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=10&snum=&e=w
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588366&imageID=1235268&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=12&snum=&e=w
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588367&imageID=1235269&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=13&snum=&e=w
http://digitalgallery.nypl.org/nypldigital/dgkeysearchdetail.cfm?trg=1&strucID=588369&imageID=1235271&total=33&num=0&parent_id=585384&s=&notword=&d=&c=&f=&k=1&sScope=&sLevel=&sLabel=&lword=&lfield=&sort=&imgs=20&pos=15&snum=&e=w