There was an awesome post in the General Section asking some stuff about the Editor by "Shnazzmeister1993".
We moved it to the Editor Section and even answered it and somehow it got deleted. Sorry dude!
So I recreate my answer here.
Yes, Under Siege is an RTT.
We started talking about our game as an RTT but everytime we did there was always the question -> "What is an RTT"? So we stoped. Now we just call it an RTS to avoid confusion.
Garrisson units in buildins?
You cannot garrisson units in buildings, they were not modelled for that.
However you can use the Logic system to technically make units disappear as if they entered teh building and put them back out when you want. It is visually not that appealing but it works. There is one or two examples of this in the Main Campaign.
Barracks spewing out units?
As you have seen in the videos we have a "Preparation Phase" where you deploy your units. No gold farming and base building envolved.
However, again like with everything else, you can turn off Spawn Points (and the Preparation Phase) on your map and create another system for unit deployment. Again this may not be visually very appealing but you can do it.