I have looked in the rules and cannot find an answer to the question posed above.
The only place on the map with over-lapping spaces are the towers
My question is: can a cannon/pole occupy the same tower as Marksman / Soldiers / Veterans?
No. One or the other but not both.
Also - what do units in a tower do? My assumption is that they can add their strength to an assault on either wall adjacent to the tower, am i correct? If so - would they support EACH adjacent wall - or only one or the other? And, can they be taken as casualties to stop a breach? Is there any other special advantage to a tower?
Marksmen on the Tower can fire at the Invaders in Assault Phase 1 Range Attack Stage 3. Their advantage is they will never be tied up in melee (like units on the wall) and so will always be able to fire. Also, they can fire at multiple ramparts opposite them.
Veterans and Soldiers don't do anything special up in the towers.