I have revised the rules sets with slightly better worded main rules and I have made the upgrades more expensive (and added an extra one):
When Orcs are determined enough to lay siege to a fortress, they employ ramshackle siege towers, to easily enable the greenskins to get over the walls.
The siege tower is flammable
When you are shooting the siege tower with missile weapons (not templates), before rolling to wound, roll a die for each hit. On the roll of a six, a stray shot gets through a gap and hits an Orc. This means that the shot is counted as hitting an Orc, and wounds apply to one of them inside the tower. Breath weapons automatically hit the crew and the tower. If the tower door is open, this roll is increased to a 4,5 or 6. Is a bolt thrower shot hits an Orc, it cannot hurt any more than one.
E.G. 20 high elf archers shoot at the tower. 11 of them hit. I then roll 11 dice, and I roll 3 sixes. This means that three Orcs are hit, and the other 8 are applied to the tower.
If you are shooting the tower with templates, then only the tower can be hit (the goblin spotter at the top manages to get the Orcs out the way)
The goblin tech that has gone into the construction of the tower is less than reliable. Every friendly turn, roll 2d6 and minus the turn number. If the total is equal to or under 2, the tower takes a single wound.
The siege tower is essentially a mobile building, and is treated exactly like a building. This means units can enter and exit, assault, and use magic on it exactly as if it was a building. It can be destroyed using the rules for destroying buildings on page 399 of the Warhammer rulebook. It has toughness 10 and 16 wounds.
If a unit of Orcs, Savage Orcs, or Black Orcs (not goblins, they are not strong enough) are garrisoned inside the tower, it can move 2d6 in the ‘remaining moves’ part of the movement phase. You pivot the engine in any direction then move 2d6 inches directly forwards. If this brings it in contact with a terrain feature or unit, then it will stop.
If it does come into contact with a unit (friend or foe) they immediately take d6 s5 hits, and must move 1 inch directly away, so it is no longer in contact with the tower.
Assault the Fortress!:
If the tower moves into contact with the fortress or another building, it stops immediately, and the siege ramp falls down. Then any unit garrisoned inside the siege tower, may assault a unit inside the charged building. If they do not win the charge, the assaulters move back inside the siege tower. If there is not an enemy unit in the charged building, the Orcs can choose to garrison inside the charged building.
If there is no unit currently garrisoned inside the siege tower when it is in contact with a building, a unit in base contact with the ladder at the back can garrison inside they can use a full movement to get inside the siege tower. If there is no unit garrisoned inside the building that has been charged as well, they may make a march move, to move into the charged building as well. You can do this as many times as you like. This represents units moving from the ground, through the siege tower, and then into the fortress.
The Siege ram may upgraded as follows:
Bolt Thrower 35pts
The siege ram may be upgraded to have a spear chukka mounted on top of the siege tower. It can fire anywhere within line of sight and in range of the spear chukka, as if it was a normal spear chukka. When the siege ramp is in the up position, it will block line of forwards.
Gobbo Archer Ports 35pts
Two night goblin archers may go inside the two side ports (two in each). Treat this as being able to fire 2 s3 shots out of each side’s port (using it’s line of site). These are fired at a bs3
Big Spikes! 35pts
The Orcs mount crude spikes onto the front of the tower. This means that they inflict 2d6 hits instead of 1d6 when they come into contact with a unit.
Boardin’ Spikes 35pts
The more clever of the Orcs have mounted frontal spikes on the bottom of the ramp, to inflict damage on defending units. When the tower charges a building, the defending unit receive d6 s6 hits, as if they were impact hits (but normal impact hits do not affect units inside buildings)