They are the most vanilla codex SM ever, reliable and strong. However the Astartes codex is not still completely represented in the original SM in Dow, hence the IF are a good chance for doing this.
I'd use the all the FOK (Medes, Meloo?) art: techmarine/tactical/assault/devastator/command squad (BB,company champion)/sternguard vet/vanguard vet/honour guard and so on, of course captain,chaplain,librarian,master of the forge and chapter master. Maybe bikes and for sure Land Raider Achilles and ironclad DN. Damocles too.
A good thing about these models is that at least for veteran sergeants you have the default banners with icons of fists, have you ever payed attention to this?
However I'd branch them like BA have been done: one branch codex-compliant and the other branch following the siege assault vanguard list made by Forgeworld.
The main differences with the standard are the lesser use of fast attack units and the addition of siege assault squads (which are virtually the assault squads but removing the jetpacks) and tactical marines with an upgrade of siege mantlets (large shield which confers them a lot of protection).
The only model that lacks and it's relevant is the centurion, which is an exosuit as a poor-man's version of the terminator armour (which is more ergonomic).
For the codex-branch it could be based on the Sentinels of Terra supplement. Nonetheless I'd rather prefer (personal choice) to keep them more impersonal to make room for all the sucessor chapters (except the ones which follow the Black templar ways) as teamcolours.
About the defensive aspect, I have seen that it's possible to have a building where units can pass through. This way I'd suggest to have a circular barricade (Aegis defense) building where marines can go in to gain extra cover (isn't DoW3 planning something like this?)
Edited by Roderick, 22 May 2016 - 02:26 PM.