Without knowing how many points you have I'll give my 2 cents:
Get 4 ticks in Supply Master for the chance to get supply back
1 tick each in Repair Mastery and Build Mastery
Full 10 ticks Defense Against Guards for the extra vitality (+2,5k health but it is going away so enjoy while we can)
Full 5 ticks in Siege Bunker (hate you ACs)
From there it is kinda up to you - good tick levels in no particular order:
If you go Cata go full 5 for the bubble
If you go Ram at least 3 ticks
ACs I like to max because I hate stealth
Full Mortar, I've killed an entire 8+ man team with the #5
Ballista I aim for 4
Full Guard Killer (not as important as Defense but still an extra 100 Power)
Siege Might is ok after getting the sweet points for more boom but a bit underwhelming.
Extra supply from Supply Capacity is always nice however VERY expensive.
Do NOT take Mercenary's Bane...waste of time till maxed out on everything else.