As everyone who reads here regularly, I'm interested in running an Ars Magica game in this time period, but wikipedia seems to indicate that Fortification in the sense we think of them really didn't start until at least a few decades later, partially in response to Viking attacks and the fall of the Carolingian Empire. (en.wikipedia.org/wiki/Castle#...h_centuries.29)
My only guess is that in this time period the Byzantines and some of the Arab Caliphates might have known how to build proper forts or castles, but that the rest Europe had lost most of that knowledge?
Much of the other sources I've found on the internet are for the later medieval ages, but not the time period I'm looking for. I was hopeful someone knows something?
The issue with this is, as you've stated, that it's potentially linked to the Carolingian Renaissance, which happens much later in the 8th century than you are implying you wish to start in. However, it's also possible that this style of architecture started during the Merovingian dynasty in the 6th century, so you may be able to go a little ahistorical here and use Pre-Romanesque architecture as the base.
In addition to this, there is also a page that has links to several rather famous (at least from a historical perspective) buildings that were erected in the 8th century:
I guess the most applicable of the structures mentioned there would be the Gormaz Castle in 746 and the belfry at Old St. Peter's Basilica in the 750s. There is also the Umayyad Palace at Jericho in 744 and the Benedictine Monastery adjoining the Basilica of Saint'Ambrogio in Milan in 739.
Granted, all of the examples I gave are from the mid-8th century, but you could also search on 7th century in architecture, which gives links to structures that went up then.
I think you can assume quite a lot of defensive structures. Most or all towns would have been fortified, as far as I can tell.
There are old city gates/walls from late roman periods still standing now; many defensive sites will have been re-built as technology improved, so wherever a castle stood later on, you could have a simpler one earlier. Roman style forts would still have existed. And there was a lot of warfare, so defenses would have been needed.
It's hard to lose the knowledge to dig a trench, use the earth excavated to make a dirt wall, and add a few sharpened sticks. Or even build stone walls. I would say that what didn't exit at the time were actual, purpose built castles, as opposed to fortified cities. There were still plenty of the latter, it's just that the need for small castles hadn't yet been felt, except maybe as fortified residences inside cities.
I would suggest you to check a recently released War game for ideas. It is an easy source for great concepts house-building techniques and fortifications.
You can easily search for youtube videos for visual pleasure.
It isn't about losing the knowledge of how to dig a ditch, but of losing the social infrastructure that gives people time to dig and maintain(!) said trench rather than eke out a harvest that doesn't quite go around, collect wood and sharpen sticks rather than have furniture, firewood and houses.
The technology for motte and bailey castles, for example, requires nothing beyond the neolithic, yet there are none in 8C Europe.
There are some hill forts (sometime repurposed from centuries earlier)...
A sturdy house is sufficient to stave off a casual assault like a raid, it takes a lot more to stave off a serious assault (strong fortifications are just the beginning), and who had the wherewithal to mount a serious assault?
This is not an era for founding towns, though many towns will have inherited their walls. That said.... en.wikipedia.org/wiki/Burh (although the new construction seems to be C9 rather than C8.)