Oh sure. Sending a big giant mech to break the siege. Such a difficult task. Take away the mech and put guns in their pilots hands. Then let's see them do combat.
Those are called skitarii and in the fluff they're quite capable of giving Space marines a hard time. Praetorians (not to be confused with the Praetorian battle servitor) being themselves, genetically and/or mechanically augmented supermen who do, on occasion wear terminator armor and have been known to have heavy weapons in place of hands and are generally the loyalist version of an obliterator.
While I'm sure the titan princeps himself would not be any great noise as a soldier, the over 1,000 skitarii that he's carrying on board will make quite an impression. Since an Imperator Titan doubles as a troop transport.
Yep. Construction is different from application.
And, again, the people that techmarines go to to learn how to maintain
and make the most effective use of equipment in the field don't know how to do it themselves?
Disagree all they want. They're main advantage are the Titans with the SM not really having anything much to take one on (though the SM don't seem to think so). Take away those big toys and they would have a hard time dealing with SM.
The
Collegia Titanica, despite the name, does not just do titans. They also command their own skitarii troops. Fluff has shown those last to be a serious threat to space marines, unlike IG. And, actually, thier main advantage would be the legio cybernetica, who unfortunately have not had TT rules for some time, but proved so effective that some were made honorary space marines by the Desert Lions chapter.
"Since the defeat of Horus the Legio Cybernetica has pledged itself anew... ...Its members now take binding oaths of loyalty more terrible than any Marine Chapter oaths." Warhammer 40k Compendium, page 97.
Yup and they haven't been successful have they? Obviously in the 40k universe, you would be one of these radicals and I of course would be on the opposite end. Better safe than have to be sorry again.
Their level of success is a matter of debate. Some of the chapters they created were successful, such as the Sons of Antaeus, who are absurdly tough even by space marine standards, with physical endurance comparable to CSMs bearing the mark of Nurgle. Others were far too mutated, such as the Flame Falcons or the Black Dragons.
What can be said for sure though is that the prospect of it raised such concerns among the great enemy that Fabius Bile made a point to personally intervene, apparently. The other factions of the Inquisition were deeply disturbed that the Thorians were so close to possibly healing the Emperor and raising him from the golden throne that they made certain to have the Grey Knights make the place extra purged once the CSMs were done.
It seems that it's a bit more reciprocal than in the case of the marines. Without the Imperium and it's soldiers the AdMech gets over run and without the AdMech the Imperium can't effectively equip it's troops. Without the Marines, the Imperium could still chuck more men at a problem, so there just isn't quite the same incentive to give marines a longer leash.
Actually, the Imperium rarely defends forge worlds with ground troops. The AdMech has proven quite capable in that under most circumstances. The Imperial/AdMech relationshi;pp is more based on the Imperium supplying raw materials and the Admech turning them into finished goods. The treaty of Mars states that the AdMech are a separate allied power, free from Imperial law within their own domains. Which is very much the same deal that was given the Ultramarines, and, one would assume, similar to the one given to any SM dominion ruled over by a 1st or 2nd founding chapter.
This has led to such conflicts as the War of the Empty Cathedrals, wherein the Church tries to prevent worlds from becoming AdMech property. Not always successfully.