Buildings that are generally applicable in both Medieval Mode and regular gameplay don't work. Medieval Mode--currently relegated to DeGroot Keep--has a very specific game imprint and buildings introduce a number of problems.
First of all, buildings inherently favor RED. RED has more access to metal and more time to work on buildings. While BLU is inside the Keep after an A/B capture, they are concentrating on capturing the final point. That would leave very little time to engage buildings built inside the higher reaches of the Keep, whereas BLU's map area is huge and immediately accessible to RED. With no objectives outside the Keep to defend and no bottlenecks to improve the effectiveness of buildings, BLU won't benefit from offensive buildings and support buildings become very vulnerable.
Buildings also form natural barriers. Even if they had no other function at all, they could be built to block access to DeGroot Keep's final point. Friendly REDs can clip through them, using them for cover, while BLUS need to either engage or circumnavigate them. The point's access is only about two dispensers wide, so a pair of Engineers could stop access to the point for at least two seconds, and unless their melee weapon swings slowly or repair rates are modulated for Medieval Mode, they could outrepair any damage done.
Sentries are generally expressed as automated crossbows. I don't know why. It's not like they're a thing. However, the common expression is slow firing rate, fire-friendly arrows. Inexplicably common, also, is the condition that Engineers must continuously hit the sentry in order to make it work. Specific stats relating to fire rate may change, but the requirement for constant Engineer supervision is mystifying--it's absolutely no fun for the Engineer. Weighed against the sniper the sentry is guaranteed to have lower potential DPS on arrows but it requires less effort so players using it would look doubly bad-they're not maximizing their damage output and they are taking the easier job. Nothing about this translates out of Medieval Mode--the sentry is too weak to justify the expensive balancing measures used to make it fair in the Keep.
Dispensers are often replaced with kegs of mead. Tables of food or healing fires show up less commonly. They favor RED for all the reasons already listed; RED has more time and resources, they offer more effective cover for RED inside the keep than they would for BLU outside. Often dispensers have no metal generation in exchange for extra healing, which obviously doesn't make any sense; only snipers, Crossbow medics and other engineers will ever need ammo, and the extra healing gives RED a big advantage when BLU is pushing for the point. Dispensers are normally fragile, but without ranged weapons enemies must expend valuable time destroying them, both to break through or to deprive the REDS of the extremely valuable boost.
Teleporters aren't that great in their traditional role. Again, they favor RED because the BLU field is not large enough to hide them or necessitate their use relative to the cost of building one. At best, a sneaky BLU engineer could build one inside the keep, but putting the effort into hiding it instead of gunning for the point is probably wasted. On the other hand, REDs can use them to potentially stall enemy Democharges on the point, or build effective telefrag traps since it's easier to guess where enemies will be when their only access point to your base is a 110HU door.
Plus, they're always generic "magic portals". Allow me to wiggle my fingers in disgust.
Ultimately, if the Engineer needs a buff in Medieval Mode, other opportunities should be examined. Most buildings offer a pretty unconditional advantage. Since Medieval Mode is already reasonably balanced (i.e. fights can go to either BLU or RED), creating new items that will mostly have appreciable benefits only for RED unbalances the game in RED's favor.
