I'm not directly familiar with Castles & Crusades, can you explain how the SIEGE Engine works? I recall hearing people say that one of the 5E DMG optional replacements for the Skills system was pretty similar.
I haven't played any of the 3 systems in question, but I'm going to put a rec for Dungeon Crawl Classics out there. A very different game that reexamine the assumptions of the genre from the ground up.
Siege engine works that you have primes and normal ability scores. With a Prime ability score you get a +8 +level to roll over (so, in general it is a 12 that you need to roll over, though you reduce that by 1 point per level, so 11 at 1st level, 10 at 2nd leve, 9 at third level) when you are trying something (for example, trying to break down a door).
[NOTE: Some would call it a +6 bonus, and say the base difficulty roll is actually an 18 rather than 20. This is actually more in line with the more recent releases of the 5e proficiency bonus if you will, putting the base DC as 18, and those with the primary ability score bonus as getting a +6 to that roll]
This can be opposed by other checks. These are also adjudicated or adjusted by level. For example, if that door you were trying to break down was being supported and refreshed by another the difficulty would go up by +1 per level.
An example of this would be if you were trying to sneak past an Guard. If you were level 5, your base difficulty to roll over would be a 7 (12 - 5). If the guard was a level 4 guard, or 4 HD guard, this would add 4 points to that 7 (7+4) which would mean you would need to roll a 4 or greater to succeed as long as Dexterity was your primary ability score.
Any ability score which is NOT your primary ability score gives you a +2 (so you would need to roll an 18 or greater as a base). So, in the example above, if dexterity was NOT your primary ability score you would need to roll a 13 at level 5 (18-5). The Guard being a 4th level guard would mean you need to roll over a 17 (13+4) to succeed.
[NOTE: In reference above, if you say 18 is the Base difficulty DC, than you don't get a bonus, so much as roll based off the DC without the proficiency bonus, only adding your level bonus instead]
Demi-humans get 2 primary ability scores, Humans get 3. Half Elves get and additional half primary ability score as one would put it, that gives them a halfway bonus between the two.
In many ways, 4e's and 5e's proficiency systems were taken from Castles and Crusades and as such are very adaptable to it (though 4e was a mere +4, and 5e adapated from that is a more gradual elevation of +2 to +6).
Similarly, they work very closely (opposed checks use proficiency bonus...etc) in how they operate.
I find Castles and Crusades actually melds even better with 5e than it did with 3e (didn't meld to well with 4e despite the proficiency system similarities though, mostly due to the extra powers each class got in 4e).
I use it in a mix and match with 5e rather often today, using more of a 5e system, but with C&C bonuses and classes as well as adventures.
