1. Get networking and multiplayer working early.
DE supports both traditional peer to peer (with optional TURN server relaying to handle problematic NAT routers), and a new client-server like mode ("host command forwarding") where all clients send their commands to the host which are then forwarded to the other clients. Age 1 uses a lockstep simulation model, except for most AI code which is only executed on the host (see here).
If you do use an off the shelf network library, test it thoroughly to help build a mental model of how it actually works (vs. how you think it works or how it's supposed to work). Develop a test app you can send the library developers to reproduce problems. If the library supports reliable in-order messaging then (at the minimum) put sequence numbers in all of your packets and assert if the library drops, reorders or duplicates packets in case there are bugs in the reliable layer.
For debugging purposes make sure all timeouts can be increased by a factor of 10x or whatever. Sometimes, debugging real-time network code is impossible in the debugger (because it inserts long pauses), so be prepared to do a lot of printf()-style debugging on multiple machines.
If you're taking an old codebase and changing it to use a new networking library or API, try to (at first) minimize the amount of changes you make to the original code. No matter how ugly it is, the original code worked, was bug fixed and shipped, and don't underestimate the value of this.
If you develop your own reliable messaging system, develop a network simulator testbed (which simulates packet loss, etc.) to automate the validation of this layer whenever it's modified and always keep it working.
Trust nothing and verify everything at multiple levels. CRC your packets, CRC the uncompressed data if you use packet compression, use session nonces in your connection-oriented layer to validate connections, validate that your reliable layer is actually reliable, etc. Make sure the initial connection process is well defined and completely understood. Everything needs timeouts of some sort and when sending unreliable messages any packet can get lost. Gaffer on Games is a great guide to this domain of problems.
Getting the game to run smoothly with X random machines across a variety of network conditions is difficult. Plan on spending a lot of time tuning the system which controls the game's turntime (command latency and sim tick rate). There are multiple sources of MP hitches (which players hate): Turntime too low (so one or more machines can't keep up with the faster ones), random CPU spikes caused by AI/pathing/etc., reliable messaging retransmit delays, random client latency spikes, AI's sending too much command data, etc. Develop strong tools to track these problems down when they occur in the field and not in your test lab.
Add cheat commands to the game to help simulate a wide range of various networking and framerate conditions.
If you send unreliable ping/pong packets to measure roundtrip client latency, filter the results because some routers are quite noisy. The statistics that go into computing the sim tick rate and turntimes should be well filtered.
Establishing the initial connections between two random machines behind NAT's is still a challenging problem - test this early.
Identify your most important packets and consider adding some form of forward error correction to them to help insulate the system from packet loss. In lockstep designs like Age, the ALL_DONE packets sent by each client to every other client to indicate end of turn are the most important and currently sent twice for redundancy. (Excluding AI's, there are no commands from the player on most turns!)
Internal testing doesn't mean much. You must have MP betas to discover the real problems. It seems virtually impossible to simulate network conditions as they occur in the wild, or the game running on customer machines. Make sure you get valuable test data back from MP betas to help diagnose problems.
Age DE's reliable messaging system is based on Brownlow's "A Reliable Messaging Protocol" in GPG 5. This is an elegant and simple NACK-based reliable protocol, except the retransmit method described in the article is not powerful enough and is sensitive to network latency (supporting only 1 packet retransmit request per roundtrip). We had to modify the system to support retransmit packets containing 64-bit bitmasks indicating which speciific packets needed to be resent.
2. Develop strong out of sync (OOS) detection tools early, and learn how to use them.
As a lockstep RTS codebase is modified you will introduce many mysterious and horrifying OOS problems. Don't let them smolder in the codebase, fix them early and fix new ones ASAP.
Functions which are not safe to use in the lockstep simulation should be marked as much. We had an accessor function which returned true if the entire map was visible, which got accidentally used in some code to determine if a building could be placed at a location. This caused OOS's whenever the user resigned (which locally exposes the entire map) and another client built walls. This little OOS took 2 days to track down.
If you are getting mysterious OOS's, you need to identify the initial cause of divergence and fix that, then repeat the OOS debugging process until no more divergences remain. Don't waste time looking at downstream effects (such as out of sync random number generators) - identify and fix that first divergence.
In Age, the original developers logged virtually everything they could in the lockstep sim. Some important events (such as where objects were being created) were left out, so we had to add unique "origin" parameters to all object creations so we knew where in the code objects were being created.
3. Do not underestimate the complexity and depth of UWP and Xbox Live development.
Your team will need at least 1-2 developers who live and breathe these platforms. These individuals are rare so you'll just have to bite the bullet and make an investment into these technologies.
4. Develop clean and defensive coding practices early on. Use static analysis, use debug heaps, pay attention to warnings, etc. Being sloppy here will increase your OOS rate and cause player and developer pain. Be smart and use every tool at your disposal.
5. Do not disable or break "old" logging code. Make sure it always compiles.
This logging code is invaluable for tracking down mysterious/rare problems and OOS's. The original developers put all this logging code in there for a reason..
6. Add debug primitives if the engine doesn't have any
This is a basic quality of life thing: You need the ability to efficiently render 2D text, debug primitives in the world, etc. If the engine doesn't support them then get them in early.
7. Profile early and make major engine architectural decisions based off actual performance metrics.
If your new renderer design relies on a specific way of rendering the game in a non-mainstream manner, then verify that your design will actually work in a prototype before betting the farm on it. Be willing to pivot to an alternate renderer design with better performance if your initial design is too slow.
Get perf. up early: Lockstep RTS multiplayer games can only tick the simulation at the rate of the slowest machine in the game. So if one machine is a dog and can only handle 20Hz, the game will feel choppy for everyone. Other major sources of perf problems like pathing or AI spikes will be obscured if rendering is running slow.
8. Figure out early on how to split up a singled threaded engine to be multithreaded.
Constraining an RTS to live on only a single thread is a recipe for performance disaster, especially if you are massively increasing the max map size and pop caps vs. the original title.
9. Many RTS systems rely on emergent behavior and are interdependent.
If you modify one of these systems, you MUST test the hell out of it before committing, and then be prepared to deal with the unpredictable downstream effects.
For example, modifying the movement code in subtle ways can break the AI, or cause it to behave suboptimally. The movement code in Age1 DE is like Starcraft's: an unholy mess. To be successful modifying code like this you must deeply understand the game and the entire system's emergent behavior.
Carelessly hacking the movement or path finding code in an RTS is akin to hacking the kernel in an OS: expect chaos.
10. Automated regression testing
The more you automate and objectify testing of movement, AI, etc. the happier your life will be and the easier you will sleep at night.
11. Playtest constantly and with enough variety
It's not enough to just play against AI's on the same map over and over. Vary it up to exercise different codepaths. You MUST playtest constantly to understand the true state of the title.
12. Assume the original developers knew what they were doing.
The old code shipped and was successful. If you don't understand it, most likely the problem is you, not the code.
For example, Age 1's original movement system has some weird code to accelerate objects as they moved downhill. This code didn't have a max velocity cap, so on very long hills units could move very quickly. We resisted modifying this code because it turns out it's a subtle but important aspect of combat on hills.
13. Don't waste time developing new templated containers and switching the engine to use them, but do reformat and clean up the old code.
Nobody will have the time to figure out your new fancy custom container classes, they'll just use std because we all know how they work.
Instead, spend that time making the old code readable so it can be enhanced without the developers going crazy trying to understand it: fix its formatting, add "m_" prefixes, etc.